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that corporation; 45 sumner street; milford, massachusetts 01757-1656; usa tel: +1 (508) 478-9200; fax: +1 (508) 478-0990; web: www.thatcorp.com that 1420, 1430 description the that 1420 and 1430 are a new genera - tion of audio differential line drivers with im- proved performance over conventional cross-coupled monolithic designs. both models exhibit low noise and distortion, high slew rate, stability under difficult loads, wide output swing, and have outputs which are short-circuit pro- tected. in addition both models incorporate patented outsmarts technology, a dual feedback-loop de - sign that prevents the excessive ground currents typical of cross-coupled output stages (ccos) when clipping into single-ended loads 1 . to overcome this problem, the that 1420 and 1430 use two individual negative-feedback loops to separately control the differential output voltage and common mode output currents, mak - ing the design inherently more stable and less sensitive to component tolerances than the ccos. most importantly, the dual-feedback design pre- vents the loss of common-mode feedback that plagues the ccos designs, avoiding the excessive ground currents and overly-distorted output waveform that can result when driving sin- gle-ended loads. where minimum output offset voltage with minimum parts count is desired, the that 1430 further improves over existing designs. in con - ventional ccos circuits, two relatively high-value electrolytic capacitors are required to reduce the offset voltage. by contrast, the that 1430 topol - ogy requires only a resistor and a single film or ceramic capacitor to achieve the same effect at lower parts count and price. that corporation outsmarts balanced line drivers features outsmarts technology tames clipping behavior into single-ended loads pin-compatible with ssm2142 balanced, floating output delivers transformer-like behavior stable when driving long cables and capacitive loads that 1430 delivers low output offset voltage using single capacitor applications differential line driver audio mix consoles distribution amplifiers audio equalizers dynamic range processors digital effects processors telecommunications systems instrumentation hi-fi equipment din+ cin+ cin- din- out- sens+ out+ sens- vcc in+ vee gnd dout- dout+ 10k 10k 50 5k 5k 10k 10k 10k 10k 20k 20k 10p 50 that 1420 c ext c ext d c aa & figure 1. that 1420 equivalent circuit diagram 1. see gary hebert?s paper, an improved balanced, floating output driver ic , presented at the 108th aes convention, feb. 2000 dip pin number so pin number 1420 pin name 1430 pin name 1 3 out- out- 2 4 sens- cap2 35gndgnd 4 6 in in 5 11 vee vee 6 12 vcc vcc 7 13 sens+ cap1 8 14 out+ out+ table 1. that 1420/1430 pin assignments
that corporation; 45 sumner street; milford, massachusetts 01757-1656; usa tel: +1 (508) 478-9200; fax: +1 (508) 478-0990; web: www.thatcorp.com page 2 that1420/1430 balanced line driver preliminary information absolute maximum ratings (t a = 25c) positive supply voltage (vcc) +18 v operating temperature range (t op ) -40 to +85 c negative supply voltage (vee) -18 v storage temperature (t st ) -40 to +150 c output short circuit duration continuous junction temperature (t j ) 150 c power dissipation (p d ) tbd mw lead temperature (t lead )(soldering 60 sec) 300 c specifications 2 2. all specifications are subject to change without notice. 3. all measurements taken with v s =18, t=25 c, unless otherwise noted electrical characteristics 3 parameter symbol conditions min. typ. max. units input impedance z in 45 k gain g1 r l =600 balanced 4.35 4.65 4.95 db single ended 4.4 4.6 4.8 db gain g2 r l =100k balanced 5.8 6 6.2 db single ended 5.8 6 6.2 db dc power supply rejection ratio psrr 4v to 18v 80 105 db output common-mode rejection ratio ocmrr f=1khz, bbc method 50 68 db output signal balance ratio sbr f=1khz, bbc method 28 40 db thd+n (balanced) thd+n 1 20hz-20khz 0.001 % 1khz 0.0005 % thd+n (single ended) thd+n 2 v o =10 v rms ,r l =600 , 20hz-20khz 0.0018 % output noise snr bal. mode, 20 khz bw -104 dbv headroom hr 0.1% thd+n 25 dbv slew rate sr 16 v/ s output common mode voltage offset v ocm r l =600 , w/o sense capacitors -300 60 300 mv that1420 v ocm r l =600 , w/ sense capacitors -6 4 6 mv output common mode voltage offset v ocm r l =600 , w/o sense capacitor -400 80 400 mv THAT1430 v ocm r l =600 , w/ sense capacitor -20 10 20 mv
theory of operation outsmarts technology the that 1420 and 1430 are similar devices, both employing the outsmarts topology, a variation of circuitry originally developed at audio toys, inc. outsmarts topology employs two negative-feedback loops -- one to control the differential signal, and a separate loop to control the common mode output levels. figures 2 and 3 show the gain core common to both the that 1420 and 1430. the gain core is a single amplifier that includes two differential input pairs, c in+/- and d in+/- , and complementary outputs, v out+ and v out-, related to each other by two gain ex - pressions, a d (s) and a c (s). the first pair of differen - tial inputs, d in+/- , are connected to the differential feedback network between the outputs and the input signal. the second differential input pair, c in+/- ,is connected to a bridge circuit which generates an er- ror signal that is used to servo the common-mode be- havior of the outputs. the loop equations are then, dd dadd out out out d in in

where a d is the differential open-loop gain, and dd dacc out out out c in in

where a c is the common-mode open-loop gain. that corporation; 45 sumner street; milford, massachusetts 01757-1656; usa tel: +1 (508) 478-9200; fax: +1 (508) 478-0990; web: www.thatcorp.com rev. 4/24/01 page 3 preliminary information electrical characteristics (cont?d.) parameter symbol conditions min. typ. max. units differential output offset v ood r l =600 -10 4 10 mv differential output voltage swing,pos v in = 18v v cc -2 v differential output voltage swing,neg v in = 18v v ee +2 v output impedance z o 40 50 60 quiescent supply current i s unloaded, v in = 0 4 5.2 ma short circuit output current i sc 60 70 ma voltage supply range 4 18 v din+ cin+ cin- din- out- sens+ out+ sens- vcc in+ vee gnd dout- dout+ 10k 10k 50 5k 5k 10k 10k 10k 10k 20k 20k 10p 50 that 1420 c ext c ext d c aa & figure 2. that 1420 equivalent circuit diagram
these equations can be solved much like standard op-amp loop equations, and for the differential case, we can see that (using superposition) resistor feed - back results in dd in in out
() 1 3 2 3 and dd in out
1 3 substituting and simplifying into the equation that defines differential operation yields da in out d d out
() 3 2 3 dividing through by a d (assuming that a d >> 3) and simplifying yields din out
2 as one would expect for a +6db line driver. the derivation for the common mode equation is more complicated 1 in that it is dependent on the at - tached load, and in any event doesn't yield much insight into the device's operation. in op-amp analysis or in the above derivation, the combination of negative feedback and high open-loop gain results in the open-loop gain "dropping out" of the equation, and the differential inputs being forced to the same potential. if we start with that assump - tion, we can intuitively discern the operation of the common-mode feedback loop as follows: referring again to figures 2 and 3, the com - mon-mode input actually senses the sum of the ic's output currents by way of two 50 ohm resistors and the bridge network (the 10pf capacitor simply limits the maximum frequency at which this action occurs). the resulting error signal is amplified and then summed into both outputs, with the net effect being to force the sum of the currents to be zero, and thus the common mode output current to zero. since this is negative feedback, the common-mode loop can raise the effective output impedance at audio fre - quencies without the side effects of circuits that use positive feedback to implement this function. that corporation; 45 sumner street; milford, massachusetts 01757-1656; usa tel: +1 (508) 478-9200; fax: +1 (508) 478-0990; web: www.thatcorp.com page 4 that1420/1430 balanced line driver preliminary information out- cap1 cap2 out+ vcc in+ vee gnd 10k 10k 50 5k 5k 10k 10k 10k 10k 7k 7k 10p 50 that 1430 c ext r ext din+ cin+ cin- din- dout- dout+ d c aa & figure 3. that 1430 equivalent circuit diagram
applications circuit implementations using the that 1420/1430 are relatively straightforward. a quiet, solid ground reference, stiff voltage supplies, and ad- equate supply bypassing are all that is required to achieve excellent performance out of both ics. both devices are stable into any capacitive load, and the maximum capacitance is limited only by slew rate and frequency response considerations. for the purposes of the frequency response calcu - lation, the line driver?s 50 sense resistors can be lumped into a single 100 resistor. the correct ca - ble capacitance to use is the sum of the inter-conductor capacitance and the two conduc - tor-to-shield capacitances. unfortunately, some man - ufacturers only specify the inter-conductor capacitance and the capacitance of one conductor to the other while connected to the shield, and some ex - traction may be required. as an example, one manufacturer supplies a shielded, twisted pair with 30pf/ft of inter-conductor capacitance and 25pf/ft of conductor to shield capac - itance. the corner frequency of the that 1420/1430 driving 500 ft of this cable will be fkhz c pf ft pf ft pf ft
1 2 100 500 30 25 25 40 () one must also consider the slew rate limitations posed by excessive cable and other capacitances. we know that ic dv dt
and that dv dt peak vf
2 rane corporation has published a document titled ranenote 126 4 , which specifies some of the require - ments for a balanced line driver, including a) stablility into reactive loads, b) output voltage swing of at least 11 volts peak (+20dbu), and c) reliabil - ity. this document also suggests a reasonable rule by which to calculate the output current require - ments at 20khz. the author concludes that the ac - that corporation; 45 sumner street; milford, massachusetts 01757-1656; usa tel: +1 (508) 478-9200; fax: +1 (508) 478-0990; web: www.thatcorp.com rev. 4/24/01 page 5 preliminary information in 4 gnd 3 5 2 out- 1 out+ 8 7 6 u1 that1420 in vcc vee 1 in- 2 in+ 3 4 7 out 6 u2 that1243 or equiv. vee vcc out c4 100n c5 100n c6 100n c7 100n vcc sens+ vee sens- vcc vee ref figure 4. basic that 1420 applications circuit 4. copyright 1991 rane corporation
tual worst case peak level for various types of music and speech will be flat out to 5khz, and roll off at 6db/octave above this frequency. thus the peak lev- els at 20khz will be 12db below those at 5khz. using these, we can calculate the required slew rate and current drive. since both outputs can swing 11v, the v peak is actually 22v (below 5khz), and at 20khz, v peak is 5.5v. therefore, dv dt v s vkhz

255 20 069 .. as a consequence, ift ma pf ft pf ft pf ft v s
500 30 25 25 0 69 28 (). thus, driving this 40nf cable requires 28ma peak (well within the 1420/1403?s capabilities). figure 4 shows the most basic connection between the that 1420 and a typical line receiver (like the that 1243). the only external components that are abso - lutely required are the local 100nf bypass capaci - tors, and these could, in fact, be shared with another nearby component. there are no common mode out - put offset reduction capacitors, and the line driver?s outputs are connected directly to their respective sense inputs. the outputs are also dc coupled to the line receiver. if large common mode voltages are ex- pected, the designer may choose to incorporate large, non-polarized capacitors to isolate the that 1420?s outputs. figure 5 shows the basic that 1430 applications circuit. this circuit includes external components for common mode offset reduction. this ic is spe - cially designed to allow common mode offset reduc - tion with only a small resistor and capacitor, and is ideal for new designs where space is at a premium. other considerations that apply to the that 1420 apply to the that 1430. figure 6 shows a that 1420 with common mode offset reduction, rfi protection and surge protection, but these last two additions could be added to the that 1430 as well. one should also note that the that 1420 is pin-for-pin compatible with industry standard line drivers. these line drivers can easily drive cables hun - dreds of feet in length without becoming unstable, but attaching such a long cable can act as an antenna (even for am stations) which can pick up rfi and di - rect it into the circuit. c3 and c8 are 100pf capaci - that corporation; 45 sumner street; milford, massachusetts 01757-1656; usa tel: +1 (508) 478-9200; fax: +1 (508) 478-0990; web: www.thatcorp.com page 6 that1420/1430 balanced line driver preliminary information in 4 gnd 3 5 2 out- 1 out+ 8 7 6 u1 THAT1430 in vcc vee c1 100n 1 in- 2 in+ 3 4 7 out 6 u2 that1243 or equiv. vee vcc out c4 100n c5 100n c6 100n c7 100n vcc cap1 vee cap2 vcc vee ref r1 1m0 figure 5. basic that 1430 application circuit with output common mode offset reduction
tors whose purpose is to redirect this rf energy to the chassis before it can circulate and effectively form a single loop transformer that magnetically couples rf into the remainder of the circuit. ferrite beads are also included to ensure that rfi current is di- rected to the chassis and not through the relatively low impedance (at rf frequencies) output of the that 1420/1430. the devices will have no effect on the gain error of these line drivers at audio frequen - cies. while both of these chips have diode protection to the rails, this protection might not be adequate for some conditions seen in the field. the most obvious problem that one might foresee would be having the line driver?s output plugged directly into a micro - phone preamplifier input that has +48v phantom power applied. this situation can result in surge currents of several amps, which can cause open cir - cuits in the metal traces or failure of the protection diodes on the ic. this circuit uses a discrete diode bridge com- posed of sb160?s to clamp potentially damaging surges to the ic?s supply rails. closing thoughts the integrated balanced line driver is one of those highly useful, cost-effective functional blocks that can provide significant improvement over discrete de - signs. the that 1420 and 1430 go a step or two further by improving over existing components. both incorporate outsmarts technology to tame the ab - errant single-ended clipping behavior of conventional cross-coupled output stages. the that 1430's de - sign gives reasonably low output offset voltage with only a resistor and a single film or ceramic capacitor, though it is not pin-compatible with existing ic out - put stages. for more information on these or other that corporation integrated circuits, please contact us di - rectly, or through one of our international distribu - tors. that corporation; 45 sumner street; milford, massachusetts 01757-1656; usa tel: +1 (508) 478-9200; fax: +1 (508) 478-0990; web: www.thatcorp.com rev. 4/24/01 page 7 preliminary information in 4 gnd 3 out- 1 out+ 8 u1 that1420 in vcc vee c1 10u c2 10u c4 100n c5 100n d3 sb160 d4 sb160 d5 sb160 d6 sb160 c3 100p c8 100p l1 ferrite bead l2 ferrite bead out hi out lo 5 6 7 2 vee sens- sens+ vcc figure 6. 1420 with output common mode offset protection, rfi protection, and surge protection
package information the that1420/1430 are available in both 8-pin mini-dip and 16-pin soic packages. the package dimensions are shown in figures 7 and 8, while pinouts are given in table 1. that corporation; 45 sumner street; milford, massachusetts 01757-1656; usa tel: +1 (508) 478-9200; fax: +1 (508) 478-0990; web: www.thatcorp.com page 8 that1420/1430 balanced line driver preliminary information b a k f h e d g j c item a b c d e f g h j k 1 millimeters 9.52 0.10 6.35 0.10 7.49/8.13 0.46 2.54 3.68/4.32 0.25 3.18 0.10 8.13/9.40 3.30 0.10 inches 0.375 0.250 0.004 0.295/0.320 0.018 0.100 0.145/0.170 0.010 0.125 0.004 0.320/0.370 0.130 0.004 0.004 figure 7. -p (dip) version package outline drawing d e bc j a f h g 1 item a b c d e f g h j millimeters 10.11/10.31 7.40/7.60 10.11/10.51 0.36/0.46 1.27 2.44/2.64 0.23/0.32 0.51/1.01 0.10/0.30 inches 0.398/o.406 0.291/0.299 0.398/0.414 0.014/0.018 0.050 0.096/0.104 0.009/0.013 0.020/0.040 0.004/0.012 figure 8. -s (so) version package outline drawing

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