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LTC1564 10kHz to 150kHz Digitally Controlled Antialiasing Filter and 4-Bit P.G.A
FEATURES
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DESCRIPTIO
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4-Bit Digitally Controlled 8th-Order Lowpass Filter - fCUTOFF Adjustable from 10kHz to 150kHz in 10kHz Steps - 100dB Attenuation at 2.5 x fCUTOFF 4-Bit Digitally Controlled Programmable Gain Amplifier - G = 1 to 16 in 1V/V Steps Miniature 16-Pin SSOP Package No External Components 122dB Total System Dynamic Range Rail-to-Rail Input and Output Range 2.7V to 10V Operation Low Noise Mute Mode Low Power Shutdown Mode
The LTC(R)1564 is a new type of continuous time filter for antialiasing, reconstruction and other band-limiting applications. No other analog components or filter expertise are needed to use it. There is one analog input pin and one analog output pin. The cutoff frequency (fC) and gain are programmable while the shape of the lowpass response is fixed. A latching digital interface stores fC and gain settings or it can be bypassed for control directly from the pins. The LTC1564 operates from 2.7V to 10V total (single or split supplies) and comes in a 16-pin surface mount SSOP. The LTC1564 is a rail-to-rail high resolution 8th-order lowpass filter with two stopband notches, giving approximately 100dB attenuation at 2.5 times the passband cutoff frequency fC (a de-facto standard for DSP front ends). Signals with low or variable levels can be normalized with the built-in variable gain that reduces input-referred noise with increasing gain for a typical dynamic range (maximum signal level to minimum noise) of 122dB (20 equivalent bits) with 20kHz fC and 118dB at 100kHz fC on a 5V supply. Other frequency-response shapes can be provided upon request. Please contact LTC Marketing.
, LTC and LT are registered trademarks of Linear Technology Corporation.
APPLICATIO S
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Antialias or Reconstruction Filtering DSP Systems Communications Systems Scientific Instruments High Resolutions (16 Bits to 20 Bits) Processing Signals Buried in Noise Audio Signal Processing Programmable Data Rates Automatic Gain Control (AGC) Single Part Replacing Multiple Filters
TYPICAL APPLICATIO
V+ 0.1F ANALOG IN 16 15 14 13
LTC1564 Programmable Range
30 20 10 0 -10 -20 -30 -40 -50 -60 fC = 10kHz -70 -80 GAIN = 1V/V -90 -100 -110 -120 10 5 fC = 150kHz GAIN = 16V/V
Low Noise Programmable Filter with Variable Gain
GAIN CODE 12 11 G2 10 G1 9 G0 V+ AND V - SUPPLIES CAN BE FROM 1.35V TO 5.25V EACH TIE F AND G PINS TO V+ OR V - TO SET FREQUENCY AND GAIN DYNAMIC RANGE 118dB TO 122dB AT 5V DEPENDING ON FREQUENCY CODE GAIN (dB)
IN AGND V + RST G3 LTC1564 OUT V - 1 2 CS/ EN HOLD F3 3 4 5
F2 6
F1 7
F0 8
0.1F ANALOG OUT V-
FREQUENCY CODE
1564 TA01
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100 FREQUENCY (kHz) 500
1564 TA02
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LTC1564
ABSOLUTE
(Note 1)
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RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW OUT V- EN CS/HOLD F3 F2 F1 F0 1 2 3 4 5 6 7 8 16 IN 15 AGND 14 V + 13 RST 12 G3 11 G2 10 G1 9 G0
Total Supply Voltage (V+ to V-) .............................. 11V Input Voltage ............................. V + + 0.3V to V - - 0.3V Output Short-Circuit Duration .......................... Indefinite Operating Temperature Range LTC1564C .............................................. 0C to 70C LTC1564I .......................................... - 40C TO 85C Storage Temperature Range ................. - 65C to 150C Lead Temperature (Soldering, 10 sec).................. 300C
ORDER PART NUMBER LTC1564CG LTC1564IG
G PACKAGE 16-LEAD PLASTIC SSOP
TJMAX = 125C, JA = 130C/ W
Consult factory for parts specified with wider operating temperature ranges.
The q denotes specifications that apply over the full operating temperature range, otherwise specifications are at TA = 25C. VS = 2.375V, fC = 10kHz, gain = 1, RL = 10k, unless otherwise noted.
PARAMETER Total Supply Voltage Supply Current VS = 1.35V, VIN = 0V VS = 2.375V, VIN = 0V VS = 5V, VIN = 0V RL = 10k to 0V VS = 5V Gain = 1, 0C to 70C Gain = 1, - 40C to 85C Gain = 10, 0C to 70C Gain = 10, - 40C to 85C VS = Single 5V Supply fC = 50kHz, fIN = 10kHz, Gain = 1 fC = 50kHz, fIN = 10KHz, Gain = 16 fC = 10kHz, 0 fIN 9kHz (Notes 2, 3) fC = 150kHz, 0 fIN 135kHz (Notes 2, 3) fC = 10kHz (F = 0001) fC = 150kHz (F = 1111) fC = 10kHz fC = 10kHz BW = 20kHz, fC = 10kHz, Gain = 1 BW = 20kHz, fC = 10kHz, Gain = 16 BW = 200kHz, fC = 100kHz, Gain = 1 fC = 100kHz, fIN = 10kHz, VIN = 1VRMS Gain = 1, DC VIN = 0V Gain = 16, DC VIN = 0V fC = 10kHz, f = 10kHz F = 0000, fIN = 20kHz, VIN = 1VRMS
q q q q q q q q q q q q q q q q
ELECTRICAL CHARACTERISTICS
CONDITIONS
MIN 2.7
TYP 15 16 22
MAX 10.5 17 18.5 25
UNITS V mA mA mA VP-P mA
Output Voltage Swing Output Short-Circuit Current DC Offset Voltage Magnitude (Referred to Input)
4.5
4.65 10 3 3 1 1 2.5 13 16 5 6 0.8 25.3 0.5 1.6 -0.7 -0.5 -62 -99 33 2.5 50 - 86 10 625 30 -103 -0.3 0.6 -59
DC AGND Reference Voltage Passband Gain Passband Ripple Roll Off at Cutoff Frequency (fC) (Note 3) Roll Off at 2fC (Note 3) Roll Off at 2.5fC (Note 3) Wideband Noise (Referred to Input)
- 0.1 23.5 -0.5 - 0.6 -1.2 -1.5 -65
0.3 24.2
VRMS VRMS VRMS dB k dB
Total Harmonic Distortion Input Impedance Output Impedance Mute State (F = 0000) Gain
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mV mV mV mV V dB dB dB dB dB dB dB dB
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LTC1564
The q denotes specifications that apply over the full operating temperature range, otherwise specifications are at TA = 25C. VS = 2.375V, fC = 10kHz, gain = 1, RL = 10k, unless otherwise noted.
PARAMETER Mute State Output Noise Shutdown Supply Current CONDITIONS F = 0000, BW = 200kHz VS = 1.35V, EN to V + VS = 1.35V, EN to V + VS = 2.375V, EN to V+ VS = 2.375V, EN to V+ VS = 5V, EN to V + (Note 4) Digital Input "High" Voltage VS = 1.35V VS = 2.375V VS = 5V VS = 1.35V VS = 2.375V VS = 5V VS = 1.35V VS = 5V VS = 1.35V VS = 5V
q q q q q
ELECTRICAL CHARACTERISTICS
MIN
TYP 5.4 45 100
MAX 75 150 150 180
UNITS VRMS A A A A A V V V
q
175 1.08 1.90 4.50 -1.08 -1.90 0.50 3.5 13 1 10 6 20 2 20
Digital Input "Low" Voltage
V V V A A A A
Digital Input Pull-Up or Pull-Down Current (Note 5) (Digital Inputs Other than EN) Digital Input Pull-Up Current (EN Input)
Note 1: Absolute Maximum Ratings are those values beyond which the life of the device may be impaired. Note 2: Response is tested in production at discrete frequencies fIN of 0.1, 0.5, 0.8 and 0.9 times fC. Note 3: Relative to gain at 0.1fC. Note 4: All digital inputs driven rail-to-rail. When driving digital inputs with 0V and 5V levels, the shutdown current will increase to 3.5mA (typ).
Note 5: Each digital input includes a small positive or negative current source to float the CMOS input to V+ or V- potential if it is unconnected. The table shows the current due to this source when the input is driven at the supply voltage opposite from the float potential. Pins CS/HOLD, F3, F2, F0 and G3 to G0 float to the V- voltage, pins RST, EN and F1 to the V+ voltage. See "Floatable Digital Inputs" in Applications Information section.
TYPICAL PERFOR A CE CHARACTERISTICS
Overall Frequency Response (Frequency Scales Normalized to fC)
10 0 -10 -20 -30 -40 -50 -60 -70 -80 -90 -100 -110 -120 -130 0.1
VS = SINGLE 5V UNITY GAIN (G CODE 0000)
GAIN (dB)
GAIN (dB)
fC = 150kHz fC = 50kHz
-5
GAIN (dB)
fC = 10kHz 1 fIN/fC 10
1564 G01
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Roll-Offs Over Temperature (fC = 10kHz)
5 fC = 10kHz SINGLE 5V SUPPLY -40C, 25C, 85C 0
0 5
Roll-Offs Over Temperature (fC = 100kHz)
fC = 100kHz SINGLE 5V SUPPLY
-40C 25C
85C
-5
-10 5 6.25 7.5 8.75 10 FREQUENCY (kHz) 11.25 12.5
-10 50 62.5 75 87.5 100 FREQUENCY (kHz) 112.5 125
1564 G02
1564 G03
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LTC1564 TYPICAL PERFOR A CE CHARACTERISTICS
Passband Roll-Off at fIN = fC vs fC
0 -0.25 -0.50 VS = SINGLE 5V TA = 25C 5 0 -5 -10 PHASE
GAIN (dB)
GAIN (dB)
-0.75 -1.00 -1.25 -1.50 10 30 50 90 70 fC (kHz) 110 130 150
Detail of Stopband Response
-40 -50 -60 -70 GAIN (dB)
2V/DIV
-80 -90
-100 -110 -120 -130 -140 15 25 45 35 FREQUENCY (kHz) 55 65
1564 G05
Triangular-Wave Time Response
fC = 10kHz fIN = 1kHz UNITY GAIN VS = 5V INPUT 5V/DIV
SIGNAL/NOISE (dB)
GAIN = 16 fC = 100kHz GAIN = 16 fC = 20kHz
(THD + NOISE)/SIGNAL (dB)
OUTPUT
200s/DIV
1564 G09
4
UW
fC = 10kHz VS = 5V TA = 25C
Passband Gain, Phase and Group Delay
fC = 10kHz GAIN 90 0 -90 -180 -270 -360 -450 GROUP DELAY -540 -630 -720 2 4 8 6 FREQUENCY (kHz) 10 12
1546 G06
500 450 400 350
PHASE (DEGREES)
DELAY (s)
-15 -20 -25 -30 -35 -40 -45
300 250 200 150 100 50 0
-810
1564 G04
Rectangular Pulse Response
fC = 10kHz UNITY GAIN VS = 5V INPUT
Short-Pulse Response
fC = 10kHz UNITY GAIN VS = 5V INPUT, 1V/DIV (PULSE WIDTH 10s)
OUTPUT, 100mV/DIV
OUTPUT
200s/DIV
1564 G07
100s/DIV
1564 G08
SNR vs Input Voltage
140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 0.001 LIMIT FOR 10V TOTAL SUPPLY LIMIT FOR 5V TOTAL SUPPLY
-20 -30
THD + Noise vs Input Voltage (fC = 10kHz)
3V SUPPLY -40 -50 -60 -70 -80 5V SUPPLY 5V SUPPLY
GAIN = 1 fC = 100kHz GAIN = 1 fC = 20kHz PASSBAND INPUT (fIN < fC)
0.1 1 0.01 INPUT VOLTAGE (VP-P)
10
1564 G10
-90 f = 10kHz C fIN = 1kHz -100 0.01 0.001 0.1 1 INPUT VOLTAGE (VP-P)
10
1564 G11
LTC1564 TYPICAL PERFOR A CE CHARACTERISTICS
THD + Noise vs Input Voltage (fC = 100kHz)
-20 3V SUPPLY -40 -50 -60 -70 -80 5V SUPPLY 5V SUPPLY
INPUT-REFERRED NOISE (VRMS)
-30
(THD + NOISE)/SIGNAL (dB)
fC = 100kHz 10 fC = 10kHz
GAIN (dB)
-90 f = 100kHz C fIN = 10kHz -100 0.01 0.001 0.1 1 INPUT VOLTAGE (VP-P)
PI FU CTIO S
OUT (Pin 1): Analog Output. In normal filtering, this is the output of an internal operational amplifier and is capable of swinging essentially to any voltage between the power supply rails (that is, between V+ and V -). This output is designed to drive a nominal load of 5k and 50pF. For lowest signal distortion it should be loaded as lightly as possible. The output can drive lower resistances than 5k, but distortion may increase, and the output current will limit at approximately 10mA. Capacitances higher than 50pF should be isolated by a series resistor of 500 to preserve AC stability. In the Mute state (F code 0000 or RST = 0), the output operates as in normal filtering but the gain from the IN pin becomes zero and the output noise is reduced. In the shutdown state (EN = 1 or EN open circuited), most of the circuitry in the LTC1564 shuts off and the OUT pin assumes a high impedance state. V -, V + (Pins 2, 14): Power Supply Pins. The V + and V - pins should be bypassed with 0.1F capacitors to an adequate analog ground plane using the shortest possible wiring. Electrically clean supplies and a low impedance ground are important for the high dynamic range and high stopband suppression available from the LTC1564 (see further details under AGND). Low noise linear power supplies are recommended. Switching supplies are not recommended because of the inevitable risk of their switching noise coupling into the signal path, reducing dynamic range. EN (Pin 3): CMOS-Level Digital Chip Enable Input. Logic 1 or open circuiting this pin causes a shutdown mode with reduced supply current. The active circuitry in the LTC1564 shuts off and its output assumes a high impedance state. If F and G bits are latched (CS/HOLD = 1) during the shutdown state, the latch will retain its contents. A small pull-up current source at the EN input causes the LTC1564 to be in shutdown state if the EN pin is left open. Therefore, the user must connect the EN pin to logic 0 (V - or optionally 0V with 5V supplies) for normal filter operation. CS/HOLD (Pin 4): CMOS-Level Digital Enable Input for the Latch Holding F and G Bits. Logic 0 makes the latch transparent so that the F and G inputs directly control the filter's cutoff frequency and gain. Logic 1 holds the last values of these inputs prior to the transition. This pin floats to logic 0 (V -) when open circuited because of a small current source (see Electrical Characteristics, Note 5). F3, F2, F1, F0 (Pins 5, 6, 7, 8): CMOS-Level Digital Frequency Control ("F Code") Inputs. F3 is the most significant bit (MSB). These pins program the LTC1564's cutoff frequency fC through the internal latch, which
UW
1564 G12
Noise vs Frequency and Gain Settings
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Power Supply Rejection vs Frequency
10 0 -10 -20 -30 -40 -50 -60 -70 POSITIVE SUPPLY V + SUPPLY BYPASS = NONE - V SUPPLY BYPASS = 0.1F 1k 10k 100k FREQUENCY (Hz) 1M
1564 G14
fC = 10kHz VS = 2.5V NEGATIVE SUPPLY V + SUPPLY BYPASS = 0.1F - SUPPLY BYPASS = NONE V
1
10
1
2 4 8 BASEBAND GAIN SETTING
16
1564 G13
-80 0.1k
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LTC1564
PI FU CTIO S
passes the bits directly when the CS/HOLD input is at logic 0. When CS/HOLD changes to logic 1, the F pins cease to have effect and the latch holds the previous values. The F code controls the filter's cutoff frequency fC in 10kHz steps up to 150kHz, as summarized in Table 1.
Table 1
F3 F2 F1 F0 (AT OUTPUT OF INTERNAL LATCH) 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 NOMINAL FC (CUTOFF FREQUENCY) 0 (Mute State: Filter Gain is Zero) 10kHz 20kHz 30kHz 40kHz 50kHz 60kHz 70kHz 80kHz 90kHz 100kHz 110kHz 120kHz 130kHz 140kHz 150kHz
Thus fC is proportional to the binary value of the F code. Note that small current sources pull F1 to V + and F3, F2 and F0 to V - when these pins are left unconnected (see Electrical Characteristics, Note 5). This sets an F code input of 0010 (2, in decimal form) by default, giving an fC of 20kHz in normal filtering operation, if CS/HOLD is logic 0 or is open circuited.
Table 2
G3 G2 G1 G0 (AT OUTPUT OF INTERNAL LATCH) 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 NOMINAL PASSBAND GAIN (VOLT/VOLT) (dB) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 0 6.0 9.5 12 14.0 15.6 16.9 18.1 19.1 20.0 20.8 21.6 22.3 22.9 23.5 24.1
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G0, G1, G2, G3 (Pins 9, 10, 11, 12): CMOS-Level Digital Gain Control ("G Code") Inputs. G3 is the most significant bit (MSB). These pins program the LTC1564's passband gain through the internal latch, which passes the bits directly when the CS/HOLD input is at logic 0. When CS/HOLD changes to logic 1, the G pins cease to have effect and the latch retains the previous input values. This gain control is linear in amplitude: nominal passband gain of the LTC1564 is the binary value of the G code, plus one as shown in Table 2. Note that small current sources pull the G pins to V - when these pins are left unconnected (see Electrical Characteristics, Note 5). This sets a G code input of 0000 by default, giving unity passband gain in normal filtering operation, if CS/HOLD is logic 0 or is open circuited. RST (Pin 13): CMOS-Level Asynchronous Reset Input. Logic 0 on this pin immediately resets the internal F and G latch to all zeros, regardless of the state of the CS/HOLD pin or the F or G input pins. This causes the LTC1564 to enter a mute state (powered but with zero signal gain) because of the resulting F = 0000 command. Logic 1 permits the other pins to control F and G. This pin floats to logic 1 (V +) when open circuited because of a small current source (see Electrical Characteristics, Note 5). A brief internal reset (shorter than the analog settling time of the filter) also occurs when power is first applied.
MAXIMUM INPUT SIGNAL LEVEL (VOLTS PEAK-TO-PEAK) DUAL 5V SINGLE 5V SINGLE 3V 10 5 3.33 2.5 2 1.67 1.43 1.25 1.1 1.0 0.91 0.83 0.77 0.71 0.67 0.63 5.0 2.5 1.67 1.25 1 0.83 0.71 0.63 0.56 0.50 0.45 0.42 0.38 0.36 0.33 0.31 3.0 1.5 1.0 0.75 0.6 0.5 0.43 0.38 0.33 0.30 0.27 0.25 0.23 0.21 0.20 0.19
NOMINAL INPUT IMPEDANCE (k) 10 5 3.33 2.5 2 1.67 1.43 1.25 1.11 1 0.91 0.83 0.77 0.71 0.66 0.63
LTC1564
PI FU CTIO S
Table 3. Summary of LTC1564 Digital Controls and Modes
EN 1 1 1 0 0 0 0 RST 1 1 0 1 0 1 1 CS/HOLD 1 0 X 0 X 1 0 F3 X X X 0 X F2 X X X 0 X F1 X X X 0 X F0 X X X 0 X G3 G2 G1 X X X X X X X X X X X X X X X X X X X X X G0 X X X X X X X FUNCTION Shutdown Mode. Filter Disabled. Latch Holds F and G Inputs Present when Last CS/HOLD = 0 Shutdown Mode. Filter Disabled. Latch Accepts F and G Inputs Shutdown Mode. Filter Disabled. Latch Contents (F and G) Reset to All Zeros Mute Mode. Filter Active, Zero Gain, Reduced Noise Mute Mode. Filter Active, Zero Gain, Reduced Noise. Latch Contents (F and G) Reset to All Zeros Normal Filtering Operation. Latch Holds F and G Inputs Present when Last CS/HOLD = 0 Normal Filtering Operation. Filter Responds Directly to F and G Input Pins (See Separate Pin Descriptions)
X = Doesn't Matter
ANALOG GROUND PLANE
SINGLE-POINT SYSTEM GROUND
Figure 1. Dual Supply Ground Plane Connection
AGND (Pin 15): Analog Ground. The AGND pin is at the midpoint of an internal resistive voltage divider, developing a potential halfway between the V+ and V- pins, with an equivalent series resistance to the pin of nominally 7k. (In the shutdown state, analog switch FETs interrupt the voltage-divider resistors and the AGND pin assumes a high impedance.) AGND also serves as the internal halfsupply reference in the LTC1564, tied to the noninverting inputs of all internal op amps and establishing the ground reference voltage for the IN and OUT pins. Because of this, very "clean" grounding is recommended, including an
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Other Than 0000 Other Than 0000
V+ 0.1F
ANALOG GROUND PLANE V+ 0.1F V +/2 REFERENCE 1F 16 15 14 13 12 11 10 9
16
15
14
13
12
11
10
9
LTC1564
LTC1564
1
2
3
4
5
6
7
8
1 2 3 4 5 6 7 8
0.1F
V-
SINGLE-POINT SYSTEM GROUND
DIGITAL GROUND PLANE (IF ANY)
1564 F01
DIGITAL GROUND PLANE (IF ANY)
1564 F01
Figure 2. Single Supply Ground Plane Connection
analog ground plane surrounding the package. For dual supply operation, this ground plane will be tied to the 0V point and the AGND pin should connect directly to the ground plane (Figure 1). For single supply operation, in contrast, if the system signal ground is at V -, the ground plane should tie to V - and the AGND pin should be ACbypassed to the ground plane by at least a 0.1F high quality capacitor (at least 1F for best AC performance) (Figure 2). As with all high dynamic range analog circuits, performance in an application will reflect the quality of the grounding.
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LTC1564
PI FU CTIO S
IN (Pin 16): Analog Input. The filter in the LTC1564 senses the voltage difference between the IN and AGND pins. In normal filtering (EN = 0, RST = 1, F code other than 0000), the IN pin connects within the LTC1564 to a digitally controlled resistance whose other end is a current-summing point at the AGND potential. At unity gain (G code 0000), the value of this input resistance is nominally 10k and the IN voltage range is rail-to-rail (V + to V-). When filtering at gain settings above unity (G code 0000), the input resistance falls as (1/gain) to nominally 625 at a gain of 16 (G code 1111) and the linear input range also falls in inverse proportion to gain. (The variable gain capability is designed to boost lower level input signals with good noise performance.) Input resistance does not vary significantly with the frequency-setting F code except in the mute state (F code 0000). In either the mute state (F code 0000 or RST = 0) or the shutdown state (EN = 1 or EN open circuited), analog switches disconnect the IN pin internally and this pin presents a very high input resistance. Circuitry driving the IN pin must be compatible with the LT1564's input resistance and with the variation of this resistance in the event that the LTC1564 is used in multiple modes. Signal sources with significant output resistance may introduce a gain error as the source's output resistance and the LTC1564's input resistance form a voltage divider. This is especially true at the higher gain or G code settings where the LTC1564's input resistance is lowest. In single supply voltage applications with elevated gain settings (G code 0000) it is important to keep in mind that the LTC1564's ground reference point is AGND, not V -. With increasing gains, the LTC1564's linear input voltage range is no longer rail-to-rail but converges toward AGND. Similarly the OUT pin swings positive or negative with respect to AGND. At unity gain (G code 0000), both IN and OUT voltages can swing from rail-torail.
BLOCK DIAGRA
IN
V+
V
-
EN
8
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VARIABLE GAIN AMPLIFIER PROGRAMMABLE FILTER V+ SHUTDOWN SWITCH R CMOS LATCH
OUT
CS/HOLD RST
R SHUTDOWN SWITCH AGND V- G3 G2 G1 G0 F3 F2 F1 F0
1564 F03
Figure 3. Block Diagram
LTC1564
APPLICATIO S I FOR ATIO
Functional Description
The LTC1564 is a self-contained, continuous time, variable gain, high order analog lowpass filter. The gain magnitude between IN and OUT pins is approximately constant for signal frequency components up to the cutoff frequency fC and falls off rapidly for frequencies above fC. The pins IN, OUT and AGND (analog ground) are the sole analog signal connections on the LTC1564; the others are power supplies and digital control inputs to select fC (and to select gain if desired). The fC range is 10kHz to 150kHz in 10kHz steps. The form of the lowpass frequency response is an 8-pole elliptic type with two stopband notches (Figure 4). This response rolls off by approximately 100dB from fC to 2.5fC. The LTC1564 is laser trimmed for fC accuracy, passband ripple, gain and offset. It delivers a combination of 100+dB stopband attenuation, 100+dB signal-to-noise ratio (SNR) and 100+kHz fC.
GAIN (dB)
100dB
fC
2.5fC
1564 F04
FREQUENCY (Hz)
Figure 4. General Shape of Frequency Response
Figure 3 is a block diagram showing analog signal path, digital control latch, and analog ground (AGND) circuitry. A proprietary active-RC architecture filters the analog signal. This architecture limits internal noise sources to near the fundamental "kT/C" bounds for a filter of this order and power consumption. The variable gain capability at the input is an integral part of the filter, and allows boosting of low level input signals with little increase in output referred noise. This permits the input noise floor to drop steadily with increasing gain, enhancing the SNR at lower signal levels. Such a property is difficult to achieve in practice by combining separate variable gain amplifier and filter circuits.
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Digital Control Logic levels for the LTC1564 digital inputs are nominally rail-to-rail CMOS. (Logic 1 is V+, logic 0 is V - or alternatively 0V with 5V supplies). The part is tested with 10% and 90% of full excursion on the inputs, thus 1.08V at 1.35V supplies, 1.9V at 2.375V and 0.5V and 4.5V at 5V. The fC and gain settings are always controlled by the output of an on-chip CMOS latch. Inputs to this latch are the pins F3 through F0, G3 through G0, the latch-enable control CS/HOLD and the asynchronous reset input RST. A logic-0 input to CS/HOLD makes the latch transparent so that the F and G input pins pass directly to the latch outputs and therefore control the filter directly. Raising CS/HOLD to logic 1 freezes the latch's output so that the F and G input pins have no effect. Logic 0 at the RST input at any time resets the latch outputs to all zeros. The all-zero state, in turn, imposes a mute mode with zero gain and low output noise if the filter is powered on (EN = 0). The all-zeros condition will persist until RST is returned to logic 1, nonzero F and G inputs are set up and the latch outputs are updated by CS/HOLD = 0. EN is a chip-enable input causing a shutdown state. Specific details on the digital controls appear in the Pin Functions section of this data sheet. Floatable Digital Inputs Every digital input of the LTC1564 includes a small current source (roughly 10A) to float the CMOS input to V+ or V- potential if the pin is unconnected. Table 4 summarizes the open-circuit default levels.
Table 4. Open-Circuit Default Input Levels
INPUT EN CS/HOLD RST F3 F2 F1 F0 G3 G2 G1 G0 FLOATING LOGIC LEVEL 1 0 1 0010 0000 EFFECT Shutdown State F and G Pins Enabled Latch Not Reset fC = 20kHz Unity Passband Gain
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Note particularly that the pull-up current source at the EN pin forces the LTC1564 to the shutdown state if this pin is left open. Therefore the user must connect EN deliberately to a logic-0 level (V -, or optionally 0V with 5V supplies) for normal filter operation. The other digital inputs float to
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LTC1564
APPLICATIO S I FOR ATIO
levels that program the part for enabled F and G pins (CS/HOLD = 0), 20kHz fC and unity passband gain. Therefore six connections (power pins, EN to logic 0, AGND, IN and OUT) are enough to set up a working 20kHz lowpass filter, and additional pins can be connected as necessary to select different fC or gain. This feature of floatable logic inputs is intended for rapid prototyping and experimentation. Floating the logic inputs is not recommended for production designs because, depending on construction details, the high impedances of these inputs may permit unwanted interference coupling and consequent erroneous digital inputs to the LTC1564. Also, it may be necessary to consider the effect of the pullup and pull-down current sources on the logic that drives the LTC1564. In particular, if the LTC1564 operates from 5V but receives digital inputs from logic using 5V and 0V, CMOS logic levels will be compatible but the possibility exists of the LTC1564 pulling current out of the driving logic at those LTC1564 inputs that are capable of floating to logic 0. That is because the small current sources at these inputs return to V -, not to 0V. If the driving logic presents a high impedance or three-state output, the LTC1564's input current may pull this output below 0V, although the current is limited to about 10A. The system designer should be aware of this possibility and ensure that any such current flow is compatible with the driving logic. Mute State The Mute mode keeps the filter powered as in normal filtering but "turns off" the signal path for minimal signal transmission (approximately -100dB) and reduced output noise. This feature may be useful for gating a signal source on and off, or for system calibration procedures. Note however that the DC output in the Mute state may shift by some millivolts compared to normal filtering because the internal signal path changes. Recovery from Mute, like other transient responses in a filter, proceeds at the time scale of the filter's pole-zero time constants and therefore is faster at the higher fC settings (that is, at the higher F codes). The LTC1564 enters the Mute state when the F bits at the latch output (Figure 3) become 0000. (It can be remem-
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bered as a "zero-bandwidth" frequency setting.) This is achieved either by presenting a 0000 code to the F inputs and lowering the CS/HOLD input to enable the latch, or alternatively at any time by lowering RST, which immediately resets the latch contents to all zeroes. Such a reset also occurs normally at the application of power, unless CS/HOLD is low and a nonzero pattern at the F inputs overrides the brief power-on reset. In the Mute state, the G gain-control inputs have no effect. Output noise in Mute is largely thermal and wideband (unlike in normal filtering, where the filter's response affects the noise spectrum). Typical Mute-state output noise is 5.4VRMS in 200kHz measurement bandwidth and less than 3VRMS in 40kHz bandwidth. It has occasionally happened elsewhere in the electronics industry that someone would characterize a circuit or system by comparing its output level in normal operation to the noise level in a Mute state as though this were a normal signalto-noise ratio (SNR), which it is not, because this signal and noise exist only at different times. A scrupulous name for such a measure is SMR, signal-to-mute ratio. Accordingly in a 40kHz bandwidth, the LTC1564 can exhibit an SMR exceeding 120dB. Construction and Instrumentation Cautions Electrically clean construction is important in applications seeking the full dynamic range or high stopband rejection of the LTC1564. Short, direct wiring will minimize parasitic capacitance and inductance. High quality supply bypass capacitors of 0.1F near the chip provide good decoupling from a clean, low inductance power source. But several inches of wire (i.e., a few microhenrys of inductance) from the power supplies, unless decoupled by substantial capacitance ( 10F) near the chip, can cause a high-Q LC resonance in the hundreds of kHz in the chip's supplies or ground reference. This may impair stopband rejection and other specifications at those frequencies. In stringent filter applications we have often found that a compact, carefully laid out printed circuit board with good ground plane makes a difference in both stopband rejection and distortion. Finally, equipment to measure filter performance can itself introduce distortion or noise floors. Checking for these limits with a wire replacing the filter is a prudent routine procedure.
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LTC1564
TYPICAL APPLICATIO S
2-Chip Flexible DSP Front End with Amplification, Antialias Filtering and A/D Conversion
2.2F 1F 10 36 AVDD 5V 1F 5V 1F
*C1 IS A 1000pF NPO, SURFACE MOUNT DEVICE PLACE AS CLOSE AS POSSIBLE TO THE LTC1608 INPUT PINS 5V 0.1F -5V 0.1F
14 3
2 249 1% METAL FILM 1 249 1% METAL FILM C1* 1 AIN+ 2 AIN-
+
INPUT
16
V+ EN V- IN
AV fC CS/ HOLD RST F 4 13 5 6 7 8 AV
LTC1564
-
OUT AGND
15 G 9 10 11 12
FILTER CONTROL ANTIALIAS FILTER/AMP ADC -5V
16-Bit Output, Sampling Rate to 500ksps, Analog Bandwidth to 150kHz, Gain to 24dB. (For More Information, See Linear Technology Magazine, May 2001)
AMPLITUDE (dB)
Boosting a 100mVRMS Input Signal to Nearly Fill the Input Range of the LTC1608 ADC. Input Frequency of 40kHz, LTC1608 fSAMPLE = 204.8ksps, LTC1564 is Set for fC = 50kHz and Gain of 16 (F = 0101, G = 1111). Measured THD is 86dB, HD2 = - 88dB, SNR = 85dB with 100mVRMS Input. Dynamic Range of Approximately 115dB
Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
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3 VREF
35 AVDD LTC1608
9 DVDD
10 DGND SHDN 33 CS 32 CONVST 31 RD 30 BUSY 27 OVDD 29 OGND 28 5V OR 3V 1F 16-BIT PARALLEL BUS 11 TO 26
1564 TA03
4 REFCOMP
7.5k 1.75X
+
22F
2.5V REF
CONTROL LOGIC AND TIMING
P CONTROL LINES
+ -
AGND 5
16-BIT SAMPLING ADC AGND 6
B15 TO B0
OUTPUT BUFFERS
D15 TO D0
DIFFERENTIAL ANALOG INPUT 2.5V
AGND 7
AGND VSS 8 34 1F
4096-Point FFT Spectrum with Low Level Input
0 -20 -40 -60 -80 -100 -120 -140 0 25.6 51.2 FREQUENCY (kHz)
1564 TA04
76.8
102.4
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LTC1564
TYPICAL APPLICATIO S
Single Supply, Very Low Noise Input Buffer for High Impedance Source Driving the Input of LTC1564 Single Supply Differential Output Driver
V+ 0.1F
V+ 0.1F 2 RSOURCE 10k VIN 3 VSOURCE OUT V - V + SUPPLY FROM 2.7V TO 10.5V TIE F AND G PINS TO V + OR GROUND TO SET FREQUENCY AND GAIN 1 VOUT 2
- +
7 LT1677 4 6 1F 16 15 14 13 GAIN CODE 12 11 G2 10 G1 9 G0
+
IN AGND V + RST G3 LTC1564 CS/ EN HOLD F3 3 4 5
IN AGND V + RST G3 LTC1564 OUT V - 1 2 CS/ EN HOLD F3 3 4 5
G2
G1
G0 3
F2 6
F1 7
F0 8
F2 6
F1 7
F0 8
1564 TA05
PACKAGE DESCRIPTIO
G Package 16-Lead Plastic SSOP (5.3mm)
(Reference LTC DWG # 05-08-1640)
5.20 - 5.38** (.205 - .212) 1.73 - 1.99 (.068 - .078) 6.07 - 6.33* (.239 - .249) 16 15 14 13 12 11 10 9
0 - 8 7.65 - 7.90 (.301 - .311) .25 - .38 (.010 - .015) .05 - .21 (.002 - .008)
.13 - .22 (.005 - .009)
.55 - .95 (.022 - .037)
.65 (.0256) BSC
1. CONTROLLING DIMENSION: MILLIMETERS 2. DIMENSIONS ARE IN MILLIMETERS (INCHES) *DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED .152mm (.006") PER SIDE **DIMENSIONS DO NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED .254mm (.010") PER SIDE
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G16 SSOP 0401
RELATED PARTS
PART NUMBER LTC1560-1 LTC1562/LTC1562-2 DESCRIPTION 1MHz/500kHz Continuous Time, Lowpass Elliptic Filter Universal 8th Order Active RC Filters COMMENTS fCUTTOFF = 500kHz or 1MHz fCUTOFF(MAX) = 150kHz (LTC1562), fCUTOFF(MAX) = 300kHz (LTC1562-2) fCUTOFF(MAX) = 256kHz 7th Order, Differential Inputs and Outputs 7th Order, Differential Input and Outputs fCLK/fCUTOFF = 64/1, fCUTOFF(MAX) = 75kHz (LTC1569-6), fCLK/fCUTOFF = 32/1, fCUTOFF(MAX) = 300kHz (LTC1569-7)
1564f LT/TP 0401 4K * PRINTED IN USA
LTC1563-2/LTC1563-3 4th Order Active RC Lowpass Filters LTC1565-31 LTC1566-1 650kHz Continuous Time, Linear Phase Lowpass Filter 2.3MHz Continuous Time Lowpass Filter
LTC1569-6/LTC1569-7 Self Clocked, 10th Order Linear Phase Lowpass Filters
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 q FAX: (408) 434-0507 q www.linear-tech.com
(c) LINEAR TECHNOLOGY CORPORATION 2001
+
V + SUPPLY FROM 4.5V TO 10.5V TIE F AND G PINS TO V + OR GROUND TO SET FREQUENCY AND GAIN. OUTPUTS DRIVE 100/1000pF LOADS
5
-
FREQUENCY CODE
FREQUENCY CODE
2.49k
2.49k
6
+
-
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V+ 0.1F
1F V VIN 16 15 14 13 GAIN CODE 12 11 10 9 2.49k 2
+
2.49k
0.1F 8 1/2 LT1813 1 VOUT+
2.49k
1/2 LT1813 4
1564 TA06
7
VOUT-

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