Greetings:
There seem to be two ways to arrange the resistors on an instrument amp input in order to satisfy the amp's DC bias current return path requirements, and thus be able to expose it's inputs to the outside world.
Method 1:
In+ o-------------+-------| \\ | |+ \\ | |- / In- o------+--------------| / | | | | \\ \\ 499k / / 499k \\ \\ / / | | | | GND GND
This way presents a 1M differential input R and 250k common mode R.
Method 2:
In+ o------+--------------| \\ | |+ \\ | |- / In- o------|---------+----| / | | \\ | / | 1M \\ | / | | | +---------+ | \\ 10k / \\ / | | GND
The second way presents a 1M diff input R and 10k common mode R.
The common mode voltage (Vcm) definition is usually given as the voltage to ground that is at the midpoint of the differential voltage (Vdm), if the differential voltage were represented by two series votlage sources of 0.5*Vdm each.
For such an arrangement, the first method is best.
However, in the real world, aren't most common mode sources more like this:
+----------\\/\\/\\------->| | + Vdm | - | +---------------------->
| \\ / \\ / | | + Vcm | - | GND
In this case, the second method is actually better, as it doesn't allow the source resistance of Vdm to convert Vcm into a differential voltage adding to Vdm and creating error in the instrument amp's output.
The second method's disadvantage of course is that it converts the input offset current of the instrument amp into a differential voltage. However, with a source resistance low enough to avoid significant error due to loading by the diff input R, most of the offset current will be shunted away by the source anyway (IF the source is grounded, not floating). But of course for a floating source, then the offset current will produce error. But for a low-gain input or unity gain typical of data acquisition inputs that simply need differential input buffering to eliminate ground loops, then the second method has advantages.
The following two LTSpice circuits allow one to play with the two topologies and realize the way in which the first method converts common mode to diff error with high source R, and the second doesn't.
Which do you prefer and why?
I suppose the argument for the first method is that if you have a floating source, then the Vcm does appear more like the definition, rather than appearing from the "bottom" of Vdm as my circuits below have it. In this case, then the first method is best.
But if you have a grounded source, then you don't need the input resistors in the first place, so there is no problem. Trouble is, for a general purpose input, you must put in the resistors and can't anticipate whether floating vs. grounded sources will be provided. In this case, perhaps the second method is best.
----------------------------------------------------------- Version 4 SHEET 1 1260 680 WIRE -64 80 -64 16 WIRE -64 304 -64 160 WIRE -64 336 -64 304 WIRE -64 448 -64 416 WIRE -64 560 -64 528 WIRE 16 16 -64 16 WIRE 272 16 96 16 WIRE 272 80 272 16 WIRE 272 304 -64 304 WIRE 272 304 272 160 WIRE 272 400 272 304 WIRE 272 560 272 480 WIRE 368 16 272 16 WIRE 368 48 368 16 WIRE 368 304 272 304 WIRE 368 304 368 256 WIRE 416 16 368 16 WIRE 416 304 368 304 WIRE 624 -96 624 -128 WIRE 624 48 368 48 WIRE 624 48 624 -16 WIRE 752 48 624 48 WIRE 752 96 752 48 WIRE 752 256 368 256 WIRE 752 256 752 144 WIRE 768 96 752 96 WIRE 768 144 752 144 WIRE 816 80 816 48 WIRE 816 192 816 160 WIRE 880 48 816 48 FLAG -64 560 0 FLAG 272 560 0 FLAG 416 16 Vpos FLAG 416 304 Vneg FLAG 816 192 0 FLAG 880 48 Vdiff FLAG 624 -128 0 SYMBOL voltage -64 432 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value {Vcm} SYMBOL voltage -64 64 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value {Vdm} SYMBOL res 0 32 R270 WINDOW 0 32 56 VTop 0 WINDOW 3 0 56 VBottom 0 SYMATTR InstName R1 SYMATTR Value 1k SYMBOL res -80 320 R0 SYMATTR InstName R2 SYMATTR Value 10 SYMBOL res 288 176 R180 WINDOW 0 36 76 Left 0 WINDOW 3 36 40 Left 0 SYMATTR InstName R3 SYMATTR Value 1Meg SYMBOL res 288 496 R180 WINDOW 0 36 76 Left 0 WINDOW 3 36 40 Left 0 SYMATTR InstName R4 SYMATTR Value 10k SYMBOL e 816 64 R0 SYMATTR InstName E1 SYMATTR Value 1000 SYMBOL current 624 -96 R0 WINDOW 0 66 -6 Top 0 WINDOW 3 82 25 Top 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName I1 SYMATTR Value {Iofs} TEXT -386 178 Left 0 !.op TEXT -8 -88 Left 0 !.params Vcm=0 Vdm=0 Iofs=450p TEXT 408 416 Left 0 ;PRO: Doesn't convert common-mode to differential with high source Z. TEXT 408 448 Left 0 ;CON: Does convert input offset current to differential input voltage.
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-------------------------------------------------------------- Version 4 SHEET 1 880 680 WIRE -960 80 -960 16 WIRE -960 224 -960 160 WIRE -960 256 -960 224 WIRE -960 368 -960 336 WIRE -960 480 -960 448 WIRE -880 16 -960 16 WIRE -704 16 -800 16 WIRE -704 336 -704 16 WIRE -704 448 -704 416 WIRE -576 -96 -576 -128 WIRE -576 16 -704 16 WIRE -576 16 -576 -16 WIRE -576 224 -960 224 WIRE -576 336 -576 224 WIRE -576 448 -704 448 WIRE -576 448 -576 416 WIRE -576 496 -576 448 WIRE -528 16 -576 16 WIRE -528 96 -528 16 WIRE -528 224 -576 224 WIRE -528 224 -528 144 WIRE -480 16 -528 16 WIRE -480 224 -528 224 WIRE -432 96 -528 96 WIRE -432 144 -528 144 WIRE -384 80 -384 48 WIRE -384 192 -384 160 WIRE -320 48 -384 48 FLAG -960 480 0 FLAG -576 496 0 FLAG -480 16 Vpos FLAG -480 224 Vneg FLAG -384 192 0 FLAG -320 48 Vdiff FLAG -576 -128 0 SYMBOL voltage -960 352 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V3 SYMATTR Value {Vcm} SYMBOL voltage -960 64 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V4 SYMATTR Value {Vdm} SYMBOL res -896 32 R270 WINDOW 0 32 56 VTop 0 WINDOW 3 0 56 VBottom 0 SYMATTR InstName R5 SYMATTR Value 1k SYMBOL res -976 240 R0 SYMATTR InstName R6 SYMATTR Value 10 SYMBOL res -688 432 R180 WINDOW 0 36 76 Left 0 WINDOW 3 36 40 Left 0 SYMATTR InstName R7 SYMATTR Value 500k SYMBOL res -560 432 R180 WINDOW 0 36 76 Left 0 WINDOW 3 36 40 Left 0 SYMATTR InstName R8 SYMATTR Value 500k SYMBOL e -384 64 R0 SYMATTR InstName E2 SYMATTR Value 1000 SYMBOL current -576 -96 R0 WINDOW 0 66 -6 Top 0 WINDOW 3 82 25 Top 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName I1 SYMATTR Value {Iofs} TEXT -994 506 Left 0 !.op TEXT -416 368 Left 0 ;CON: Does convert common-mode to differential with high source Z. TEXT -416 400 Left 0 ;PRO: Doesn't convert input offset current to differential input voltage. TEXT -1024 -144 Left 0 !.params Vcm=1 Vdm=0 Iofs=0
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Good day!