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A
FEATURES SC70 Package Very Low IB: 1 pA Max Single-Supply Operation: Dual-Supply Operation: Rail-to-Rail Output Low Supply Current: 630 Low Offset Voltage: 500 Unity Gain Stable No Phase Reversal 5 V to 26 V 2.5 V to 13 V A Typ V Max
Precision Low Power Single-Supply JFET Amplifier AD8627*
PIN CONFIGURATIONS 5-Lead SC70 (KS Suffix)
OUT A V- +IN 1 2 3 5 V+
AD8627
4 -IN
APPLICATIONS Photodiode Amplifier ATE Line Powered/Battery Powered Instrumentation Industrial Controls Automotive Sensors Precision Filters Audio
8-Lead SOIC (RN Suffix)
NC -IN +IN V- 1 2 8 NC 7 V+
AD8627
3 4 6 OUT 5 NC
NC = NO CONNECT
GENERAL DESCRIPTION
The AD8627 is a precision JFET input amplifier. It features true single-supply operation, low power consumption, and rail-to-rail output. The outputs remain stable with capacitive loads of over 500 pF; the supply current is less than 630 A. Applications for the AD8627 include photodiode transimpedance amplification, ATE reference level drivers, battery management, both line powered and portable instrumentation, and remote sensor signal conditioning including automotive sensors. The AD8627's ability to swing nearly rail-to-rail at the input and rail-to-rail at the output enables it to be used to buffer CMOS DACs, ASICs, and other wide output swing devices in singlesupply systems. The 5 MHz bandwidth and low offset are ideal for precision filters. The AD8627 is fully specified over the extended industrial (-40C to +125C) temperature range and is available in both 5-lead SC70 and 8-lead SOIC surface mount packages. The SC70 packaged parts are available in tape and reel only.
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Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Trademarks and registered trademarks are the property of their respective companies.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781/329-4700 www.analog.com Fax: 781/326-8703 (c) 2003 Analog Devices, Inc. All rights reserved.
AD8627-SPECIFICATIONS
ELECTRICAL CHARACTERISTICS (@V = 5 V, V
S CM
= 1.5 V, TA = 25 C, unless otherwise noted.)
Min Typ 0.05 Max 0.5 1 1 500 0.5 25 3 Unit mV mV pA pA pA pA V dB V/mV V/C V V V V mA dB A A V/s MHz Degrees V p-p nV/Hz fA/Hz
Parameter INPUT CHARACTERISTICS Offset Voltage Input Bias Current Input Offset Current Input Voltage Range Common-Mode Rejection Ratio Large Signal Voltage Gain Offset Voltage Drift OUTPUT CHARACTERISTICS Output Voltage High Output Voltage Low Output Current POWER SUPPLY Power Supply Rejection Ratio Supply Current DYNAMIC PERFORMANCE Slew Rate Gain Bandwidth Product Phase Margin NOISE PERFORMANCE Voltage Noise Voltage Noise Density Current Noise Density
Specifications subject to change without notice.
Symbol VOS
Conditions
-40C < TA < +125C IB -40C < TA < +125C IOS -40C < TA < +125C CMRR AVO VOS/T VOH IL = 2 mA, -40C < TA < +125C VOL IL = 2 mA, -40C < TA < +125C IOUT PSRR ISY VS = 5 V to 26 V -40C < TA < +125C SR GBP OO en p-p en in 0.1 Hz to 10 Hz f = 1 kHz f = 1 kHz 5 5 60 1.9 17.5 0.4 80 10 104 630 VCM = 0 V to 2.5 V RL = 10 k, VO = 0.5 V to 4.5 V -40C < TA < +125C 0 66 100 87 230 2.5 0.25
4.96 4.90 0.04 0.08
800 800
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AD8627 ELECTRICAL CHARACTERISTICS (@V =
S
13 V, VCM = 0 V, TA = 25 C, unless otherwise noted.)
Min Typ 0.35 Max 0.75 1.75 1 500 0.5 25 +11 Unit mV mV pA pA pA pA V dB V/mV V/C V V V V mA dB A A V/s MHz Degrees V p-p nV/Hz fA/Hz
Parameter INPUT CHARACTERISTICS Offset Voltage Input Bias Current Input Offset Current Input Voltage Range Common-Mode Rejection Ratio Large Signal Voltage Gain Offset Voltage Drift OUTPUT CHARACTERISTICS Output Voltage High Output Voltage Low Output Current POWER SUPPLY Power Supply Rejection Ratio Supply Current DYNAMIC PERFORMANCE Slew Rate Gain Bandwidth Product Phase Margin NOISE PERFORMANCE Voltage Noise Voltage Noise Density Current Noise Density
Specifications subject to change without notice.
Symbol VOS
Conditions
-40C < TA < +125C IB -40C < TA < +125C IOS -40C < TA < +125C CMRR AVO VOS/T VOH VOH VOL VOL IOUT PSRR ISY VCM = -13 V to +10 V RL = 10 k, VO = -11 V to +11 V -40C < TA < +125C -13 76 150 105 310 2.5 0.25
IL = 2 mA, -40C to +125C IL = 2 mA, -40C to +125C
+12.96 +12.92 -12.96 -12.92 15 80 104 710
VS = 2.5 V to 13 V -40C < TA < +125C
900 900
SR GBP OO en p-p en in 0.1 Hz to 10 Hz f = 1 kHz f = 1 kHz
5 5 60 2.5 16 0.5
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AD8627
ABSOLUTE MAXIMUM RATINGS*
Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 V Input Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VS- to VS+ Differential Input Voltage . . . . . . . . . . . . . . . Supply Voltage Output Short Circuit Duration . . . . . . . . . . . . . . . . Indefinite Storage Temperature Range RN Package . . . . . . . . . . . . . . . . . . . . . . . -65C to +125C Operating Temperature Range . . . . . . . . . . -40C to +125C Junction Temperature Range RN Package . . . . . . . . . . . . . . . . . . . . . . . -65C to +150C Lead Temperature Range (Soldering, 60 sec) . . . . . . . . 300C
*Absolute maximum ratings apply at 25C, unless otherwise noted.
Package Type 5-Lead SC70 (KS) 8-Lead SOIC (RN)
JA*
JC
Unit C/W C/W
376 158
126 43
*JA is specified for worst case conditions, i.e., JA is specified for device soldered in circuit board for surface mount packages.
ORDERING GUIDE
Model AD8627AKS-Reel AD8627AKS-Reel7 AD8627AR AD8627AR-Reel AD8627AR-Reel7
Temperature Range -40C to +125C -40C to +125C -40C to +125C -40C to +125C -40C to +125C
Package Description 5-Lead SC70 5-Lead SC70 8-Lead SOIC 8-Lead SOIC 8-Lead SOIC
Package Option KS-5 KS-5 RN-8 RN-8 RN-8
Branding Information B9A B9A
CAUTION ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on the human body and test equipment and can discharge without detection. Although the AD8627 features proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance degradation or loss of functionality.
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Typical Performance Characteristics-AD8627
25 VSY = 13V TA = 25 C 20
NUMBER OF AMPLIFIERS NUMBER OF AMPLIFIERS
16 VSY = +3.5V/-1.5V 14 12 10 8 6 4 2
15
10
5
0
-600
-400
-200
0 VOLTAGE -
200 V
400
600
0
0
1
2
3 4 5 7 6 OFFSET VOLTAGE - V/ C
8
9
10
TPC 1. Input Offset Voltage
TPC 4. Offset Voltage Drift
12 VSY = 13V 10
50 40 VSY = 13V TA = 25 C
INPUT BIAS CURRENT - pA
0 1 2 3 4 5 6 7 OFFSET VOLTAGE - V/ C 8 9 10
30 20 10 0 -10 -20 -30 -40
NUMBER OF AMPLIFIERS
8
6
4
2
0
-50 -15 -12.5 -10 -7.5 -5 -2.5 0 2.5 VCM - V
5
7.5
10 12.5 15
TPC 2. Offset Voltage Drift
TPC 5a. Input Bias Current vs. VCM
18 VSY = +3.5V/-1.5V 16
INPUT BIAS CURRENT - pA
0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 -0.8
-400 -300 -200 -100 VOLTAGE - 0 V 100 200 300
VSY = 13V TA = 25 C
NUMBER OF AMPLIFIERS
14 12 10 8 6 4 2 0
-0.9 -15 -12.5 -10 -7.5 -5 -2.5 0 2.5 VCM - V
5
7.5
10 12.5 15
TPC 3. Input Offset Voltage
TPC 5b. Input Bias Current vs. VCM
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AD8627
100 VSY = 13V VCM = 0V INPUT BIAS CURRENT - pA
500 VSY = 5V 400
V INPUT OFFSET VOLTAGE -
300 200 100 0 -100 -200 -300 -400
10
1
0.1 -50
-25
0
25 50 75 TEMPERATURE - C
100
125
150
-500 -1
0
1 VCM - V
2
3
4
TPC 6. Input Bias Current vs. Temperature
TPC 9. Input Offset Voltage vs. VCM
2.0 VSY = +5V OR 5V 1.5
INPUT BIAS CURRENT - pA
10M
OPEN-LOOP GAIN - V/V
1.0 0.5 0 -0.5 -1.0 -1.5 -2.0 -5
1M VSY = 13V VSY = +5V 100k
-4
-3
-2
-1
0 1 VCM - V
2
3
4
5
10k 0.1
1 10 LOAD RESISTANCE - k
100
TPC 7. Input Bias Current vs. VCM
TPC 10. Open-Loop Gain vs. Load Resistance
1,000 VSY = 13V 900
V
1000 a d
OPEN-LOOP GAIN - mV/V
800 700 600 500 400 300 200 100 0 -100 -15 -12 -9 -6 -3 0 3 VCM - V 6 9 12 15
INPUT OFFSET VOLTAGE -
b 100 c e
10 a. V SY = 13V, VO = 11V, RL = 10k b. V SY = 13V, VO = 11V, RL = 2k c. V SY = +5V, VO = +0.5V/+4.5V, RL = 2k d. V SY = +5V, VO = +0.5V/+4.5V, RL = 10k e. V SY = +5V, VO = +0.5V/+4.5V, RL = 600 1 -40 25 85 TEMPERATURE - C 125
TPC 8. Input Offset Voltage vs. VCM
TPC 11. Open-Loop Gain vs. Temperature
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AD8627
600 VSY = 13V 500
10k VSY = 13V
OFFSET VOLTAGE - V
RL = 10k 300 200 RL = 100k 100 0 -100 -200 -300 -400 -15 -10 -5 0 OUTPUT VOLTAGE - V 10 15 RL = 600
VSY - OUTPUT VOLTAGE - mV
400
1k
100
VOL 10 VOH
1 0.001
0.01
0.1 1 LOAD CURRENT - mA
10
100
TPC 12. Input Error Voltage vs. Output Voltage for Resistive Loads
TPC 15. Output Saturation Voltage vs. Load Current
250 RL = 1k 200 POS RAIL RL = 10k
V
10k
VSY = 5V
VSY = 5V
RL = 100k
VSY - OUTPUT VOLTAGE - mV
150 100 50 0 RL = 10k -50 RL = 1k -100 NEG RAIL -150 -200 -250 0 200 50 100 150 250 OUTPUT VOLTAGE FROM SUPPLY RAILS - mV 300
1k
INPUT VOLTAGE -
100
VOL 10 VOH
1 0.001
0.01
0.1 1 LOAD CURRENT - mA
10
100
TPC 13. Input Error Voltage vs. Output Voltage within 300 mV of Supply Rails
TPC 16. Output Saturation Voltage vs. Load Current
800 700 +125 C
A
70 60 50
-55 C
315 VSY = 13V RL = 2k CL = 40pF 270 225
PHASE - Degrees
600
QUIESCENT CURRENT -
40 GAIN
GAIN - dB
180 135 90 PHASE 45 0 -45 -90 100k 1M FREQUENCY - Hz 10M
500 400 300 200
+25 C
30 20 10 0 -10
100 0
-20
0 4 8 12 16 20 TOTAL SUPPLY VOLTAGE - V 24 28
-30 10k
-135 50M
TPC 14. Quiescent Current vs. Supply Voltage at Different Temperatures
TPC 17. Open-Loop Gain and Phase Margin vs. Frequency
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AD8627
70 60 50 40 315 VSY = 5V RL = 2k 270 CL = 40pF 225 140 VSY = 13V 120 100 80
PHASE - Degrees
180 135 90 45 0 -45 -90 100k 1M FREQUENCY - Hz 10M
GAIN - Units
CMRR - dB
30 20 10 0 -10 -20 -30 10k
60 40 20 0 -20 -40 -60 1k 10k 100k FREQUENCY - Hz 1M 10M
-135 50M
TPC 18. Open-Loop Gain and Phase Margin vs. Frequency
TPC 21. CMRR vs. Frequency
70 60 50 40 G = 100
CMRR - dB
GAIN - dB
140 VSY = 13V RL = 2k CL = 40pF VSY = 5V 120 100 80 60 40 20 0 G=1 -20 -40 1k 10k 100k 1M FREQUENCY - Hz 10M 50M -60 1k 10k 100k FREQUENCY - Hz 1M 10M
30 20 G = 10 10 0
-10 -20 -30
TPC 19. Closed-Loop Gain vs. Frequency
TPC 22. CMRR vs. Frequency
70 60 50 40 G = 100
GAIN - dB
140
VSY = 5V RL = 2k CL = 40pF
VSY = 13V 120 100 80 +PSRR PSRR - dB 60 40 -PSRR 20 0
30 20 G = 10 10 0 G=1
-10 -20 -30 1k 10k 100k 1M FREQUENCY - Hz 10M 50M
-20 -40 -60 1k 10k 100k FREQUENCY - Hz 1M 10M
TPC 20. Closed-Loop Gain vs. Frequency
TPC 23. PSRR vs. Frequency
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AD8627
140 VSY = 5V 120 100
VOLTAGE - 10V/DIV
INPUT
VSY = 13V
80
PSRR - dB
60 40 +PSRR 20 -PSRR 0 -20 -40 -60 1k 10k 100k FREQUENCY - Hz 1M 10M
OUTPUT
TIME - 400 s/DIV
TPC 24. PSRR vs. Frequency
TPC 27. No Phase Reversal
300 VSY = 13V 270 240
OUTPUT SWING - V
15
10 TS + (1%) 5 TS + (0.1%) 0 TS - (0.1%) -5 TS - (1%) -10
210 180
ZOUT -
150 120 90 G = 10 60 G = 100 30 0 1k 10k 100k 1M FREQUENCY - Hz 10M 100M G=1
-15
0
0.5
1 1.5 SETTLING TIME - s
2
2.5
TPC 25. Output Impedance vs. Frequency
TPC 28. Output Swing and Error vs. Settling Time
300 VSY = 5V 270 240
40 35 30 VSY = 13V RL = 10k VIN = 100mV p-p OS- OS+ 20 15 10 5 0 10
210
OVERSHOOT - %
180
ZOUT -
25
150 120 90 G = 10 60 G = 100 30 0 1k 10k 100k 1M FREQUENCY - Hz 10M 100M G=1
100 CAPACITANCE - pF
1k
TPC 26. Output Impedance vs. Frequency
TPC 29. Small Signal Overshoot vs. Load Capacitance
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AD8627
60 VSY = 5V RL = 10k VIN = 100mV p-p OS-
53 VSY = 13V 46 39
VOLTAGE - nV
50
OVERSHOOT - %
40
32 19.7nV/ Hz 28 21 14
30 OS+ 20
10
7
0 10
100 CAPACITANCE - pF
1k
0 0 1 2 3 4 5 6 FREQUENCY - kHz 7 8 9 10
TPC 30. Small Signal Overshoot vs. Load Capacitance
TPC 33. Voltage Noise Density
56
VSY = 13V AVO = 100,000V/V
VSY = 5V 49 42
VOLTAGE - 50mV/DIV
VOLTAGE - nV
35 16.7nV/ Hz 28 21 14 7 0
0
0
1
2
3
TIME - 1s/DIV
4 5 6 FREQUENCY - kHz
7
8
9
10
TPC 31. 0.1 Hz to 10 Hz Noise
TPC 34. Voltage Noise Density
-40
VSY = 2.5V AVO = 100,000V/V
-50
VOLTAGE - 50mV/DIV
-60
NOISE - dB
-70 VSY = 5V, V IN = 9V p-p -80 VSY = 13V, V IN = 18V p-p
0
-90 VSY = 2.5V, V IN = 4.5V p-p
-100 -110 20
TIME - 1s/DIV
100
1k FREQUENCY - Hz
10k
100k
TPC 32. 0.1 Hz to 10 Hz Noise
TPC 35. Total Harmonic Distortion + Noise vs. Frequency
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AD8627
APPLICATIONS
The AD8627 is one of the smallest and lowest cost JFETs offered. It has true single-supply capability and has an input voltage range that extends below the negative rail, allowing the part to accommodate input signals below ground. The rail-to-rail output of the AD8627 provides the maximum dynamic range in many applications. The AD8627 uses n-channel JFETs to provide a low offset, low noise, high impedance input stage. The input common-mode voltage extends from 0.2 V below -VS to 2 V below +VS. Driving the input of the amplifier, configured in unity gain buffer, closer than 2 V to the positive rail will cause an increase in commonmode voltage error as illustrated in TPC 13 and a loss of amplifier bandwidth. This loss of bandwidth causes the rounding of the output waveforms shown in Figures 1a and 1b, which have inputs that are 1 V and 0 V from +VS, respectively.
VSY = 5V INPUT
VOLTAGE - 2V/DIV
The AD8627 can safely withstand input voltages 15 V below -VSY, as long as the total voltage between the positive supply and the input terminal is less than 26 V. Figures 2a, 2b, and 2c show the AD8627 in different configurations accommodating signals close to the negative rail. The amplifier input stage typically maintains picoamp level input currents across that input voltage range.
20k +5V 10k 0V -2.5V
VSY = 5V, 0V
OUTPUT
VOLTAGE - 1V/DIV
TIME - 2 s/DIV
TIME - 2 s/DIV
Figure 2a. Gain of Two Inverter Response to 2.5 V Step, Centered -1.25 V Below Ground
60mV
Figure 1a. Unity Gain Follower Response to 0 V to 4 V Step
VSY = 5V INPUT
20mV 0V
5V
600
VOLTAGE - 2V/DIV
OUTPUT
VOLTAGE - 1V/DIV
TIME - 2 s/DIV
Figure 1b. Unity Gain Follower Response to 0 V to 5 V Step
The AD8627 will not experience phase reversal with input signals close to the positive rail, as shown in TPC 27. For input voltages greater than +VSY, a resistor in series with the AD8627's noninverting input will prevent phase reversal at the expense of greater input voltage noise. This current limiting resistor should also be used if there is a possibility of the input voltage exceeding the positive supply by more than 300 mV, or if an input voltage will be applied to the AD8627 when VSY = 0. Either of these conditions will damage the amplifier if the condition exists for more than 10 seconds. A 100 k resistor allows the amplifier to withstand up to 10 V of continuous overvoltage, while increasing the input voltage noise by a negligible amount. REV. 0 -11-
VSY = 5V RL = 600
TIME - 2 s/DIV
Figure 2b. Unity Gain Follower Response to 40 mV Step, Centered 40 mV Above Ground
AD8627
20k +5V 10k 0V -10mV -30mV
Minimizing Input Current
VSY = 5V
VOLTAGE - 10mV/DIV
The AD8627 is guaranteed to 1 pA max input current with a 13 V supply voltage at room temperature. Careful attention to how the amplifier is used will maintain or possibly better this performance. The amplifier's operating temperature should be kept as low as possible. Like other JFET input amplifiers, the AD8627's input current will double for every 10C rise in junction temperature, as illustrated in TPC 6. On-chip power dissipation will raise the device operating temperature, causing an increase in input current. Reducing supply voltage to cut power dissipation will reduce the AD8627's input current. Heavy output loads can also increase chip temperature; maintaining a minimum load resistance of 1 k is recommended. The AD8627 is designed for mounting on PC boards. Maintaining picoampere resolution in those environments requires a lot of care. Both the board and the amplifier's package have finite resistance. Voltage differences between the input pins and other pins as well as PC board metal traces will possibly cause parasitic currents larger than the AD8627's input current unless special precautions are taken. For proper board layout where you can get the best result, refer to the ADI website for proper layout seminar material. Two common methods of minimizing parasitic leakages that should be used are guarding of the input lines and maintaining adequate insulation resistance. Contaminants such as solder flux on the board's surface and the amplifier's package can greatly reduce the insulation resistance between the input pin and those traces with supply or signal voltages. Both the package and the board must be kept clean and dry.
Photodiode Preamplifier Application
TIME - 2 s/DIV
Figure 2c. Gain of Two Inverter Response to 20 mV Step, Centered 20 mV Below Ground
The AD8627 is designed for 16 nV/Hz wideband input voltage noise and maintains low noise performance to low frequencies, as shown in TPC 33. This noise performance, along with the AD8627's low input current and current noise, means that the AD8627 contributes negligible noise for applications with large source resistances. The AD8627 has a unique bipolar rail-to-rail output stage that swings within 5 mV of the rail when up to 2 mA of current is drawn. At larger loads, the drop-out voltage increases as shown in TPC 15 and 16. The AD8627's wide bandwidth and fast slew rate allows it to be used with faster signals than previous singlesupply JFETs. Figure 3 shows the response of AD8627, configured in unity gain, to a VIN of 20 V p-p at 50 kHz. The FPBW of the part is close to 100 kHz.
VSY = 13V RL = 600
The low input current and offset voltage levels of the AD8627, together with its low voltage noise, make this amplifier an excellent choice for preamplifiers used in sensitive photodiode applications. In a typical photovoltaic preamp circuit, shown in Figure 4, the output of the amplifier is equal to: VOUT = - ID(Rf ) = -Rp(P )Rf where ID = photodiode signal current (A) Rp = photodiode sensitivity (A/W) Rf = value of the feedback resistor, in P = light power incident to photodiode surface, in W The amplifier's input current, IB, will contribute an output voltage error that will be proportional to the value of the feedback resistor. The offset voltage error, VOS, will cause a small current error due to the photodiode's finite shunt resistance, RD. The resulting output voltage error, VE, is equal to:
Rf VE = 1 + VOS + Rf (I B ) RD
VOLTAGE - 5V/DIV
TIME - 5 s/DIV
Figure 3. Unity Gain Follower Response to 20 V, 50 kHz Input Signal
A shunt resistance on the order of 100 M is typical for a small photodiode. Resistance RD is a junction resistance that will typically drop by a factor of two for every 10C rise in temperature. In the AD8627, both the offset voltage and drift are low, which helps minimize these errors. With IB values of 1 pA and VOS of 50 mV, VE for Figure 4 is very negligible. Also the circuit in Figure 4 results in an SNR value of 95 dB for a signal bandwidth of 30 kHz.
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AD8627
CF 5pF
0.1 F 5V 2.5V 10 F 0.1 F 5V SERIAL INTERFACE VDD CS DIN SCLK VREFF* VREFS*
PHOTODIODE
VOS C4 I 15pF B
RF 1.5M
RD 100M
IB
AD8627
OUTPUT
AD5551/AD5552
OUT
AD8627
UNIPOLAR OUTPUT
Figure 4. A Photodiode Model Showing DC Error
Output Amplifier for Digital-to-Analog Converters (DACs)
LDAC* DGND *AD5552 ONLY
AGND
Many system designers use amplifiers as buffers on the output of amplifiers to increase the DAC's output driving capability. The high resolution current output DACs need high precision amplifiers on their output as current to voltage converters (I/V). Additionally, many DACs operate with a single supply of 5 V. In a single-supply application, selection of a suitable op amp may be more difficult as the output swing of the amplifier does not usually include the negative rail, in this case AGND. This can result in some degradation of the DAC's specified performance unless the application does not use codes near zero. The selected op amp needs to have very low offset voltage--for a 14-bit DAC, the DAC LSB is 300 V with a 5 V reference--to eliminate the need for output offset trims. Input bias current should also be very low as the bias current multiplied by the DAC output impedance (around 10 k in some cases) will add to the zero code error. Rail-to-rail input and output performance is desired. For fast settling, the slew rate of the op amp should not impede the settling time of the DAC. Output impedance of the DAC is constant and code independent, but in order to minimize gain errors, the input impedance of the output amplifier should be as high as possible. The AD8627, with very high input impedance, IB of 1 pA, and fast slew rate, is an ideal amplifier for these types of applications. A typical configuration with a popular DAC is shown in Figure 5. In these situations, the amplifier adds another time constant to the system, increasing the settling time of the output. The AD8627, with 5 MHz of BW, helps in achieving a faster effective settling time of the combined DAC and amplifier.
Figure 5. Unipolar Output
In applications with full four-quadrant multiplying capability or a bipolar output swing, the circuit in Figure 6 can be used. In this circuit, the first and second amplifiers provide a total gain of 2, which increases the output voltage span to 20 V. Biasing the external amplifier with a 10 V offset from the reference voltage results in a full four-quadrant multiplying circuit.
10k
10k
+13V
10V VREF AD668
5k
AD8627
VOUT
-10V < V OUT < +10V -13V VDD VREFX ONE CHANNEL AD5544 VSS AGNDF AGNDX RFBX
+13V
AD8627
DIGITAL INTERFACE CONNECTIONS OMITTED FOR CLARITY
-13V
Figure 6. Four-Quadrant Multiplying Application Circuit
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AD8627
Eight-Pole Sallen Key Low-Pass Filter
1.2 V4 V2 0.8
VOLTAGE - V
The AD8627's high input impedance and dc precision make it a great selection for active filters. Due to the very low bias current of the AD8627, high value resistors can be used to construct low frequency filters. The AD8627's picoamp level input currents contribute minimal dc errors. Figure 7 shows an example, a 10 Hz eight-pole Sallen Key Filter constructed using the AD8627. Different numbers of the AD8627 can be used depending on the desired response, which is shown in Figure 8. The high value used for R1 minimizes interaction with signal source resistance. Pole placement in this version of the filter minimizes the Q associated with the lower pole section of the filter. This eliminates any peaking of the noise contribution of resistors in the preceding sections, minimizing the inherent output voltage noise of the filter.
V3
V1
0.4
0 0.1
1
10 FREQUENCY - Hz
100
1k
Figure 8. Frequency Response Output at Different Stages of the Low-Pass Filter
R1 162.3k V3 VIN D C2 96.19 F D R2 162.3k C1 100 F
VDD 3 2 1 VEE R3 25k C4 69.14 F D U1 7 5 R10 V1 191.4k R5 191.4k C3 100 F VDD 3 2 1 VEE R4 25k R7 286.5k 3 2 C6 30.86 F D 1 VEE R6 25k C8 3.805 F D VDD U3 7 5 V3 R12 815.8k R9 815.8k C7 100 F VDD 3 2 1 VEE R8 25k U4 7 5 V4 U2 7 5 V2 R11 286.5k
C5 100 F
Figure 7. 10 Hz, Eight-Pole Sallen Key Low-Pass Filter
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AD8627
OUTLINE DIMENSIONS
5-Lead Plastic Surface Mount Package [SC70] (KS-5)
Dimensions shown in millimeters
2.00 BSC
8-Lead Standard Small Outline Package [SOIC] Narrow Body (RN-8)
Dimensions shown in millimeters and (inches)
5.00 (0.1968) 4.80 (0.1890)
5
4
1.25 BSC
1 2 3
2.10 BSC
8
5 4
4.00 (0.1574) 3.80 (0.1497)
1
6.20 (0.2440) 5.80 (0.2284)
PIN 1 1.00 0.90 0.70 0.65 BSC 1.10 MAX 0.22 0.08 0.30 0.15 0.10 COPLANARITY SEATING PLANE 0.46 0.36 0.26 0.25 (0.0098) 0.10 (0.0040) COPLANARITY SEATING 0.10 PLANE 1.27 (0.0500) BSC 1.75 (0.0688) 1.35 (0.0532) 8 0.25 (0.0098) 0 0.19 (0.0075) 0.50 (0.0196) 0.25 (0.0099) 45
0.10 MAX
0.51 (0.0201) 0.33 (0.0130)
1.27 (0.0500) 0.41 (0.0160)
COMPLIANT TO JEDEC STANDARDS MO-203AA
COMPLIANT TO JEDEC STANDARDS MS-012AA CONTROLLING DIMENSIONS ARE IN MILLIMETERS; INCH DIMENSIONS (IN PARENTHESES) ARE ROUNDED-OFF MILLIMETER EQUIVALENTS FOR REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN
REV. 0
-15-
-16-
C03023-0-2/03(0)
PRINTED IN U.S.A.

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