AD8538 as auto-zero buffer?

I've done a bit of reading on the operating principle behind the auto-zero amplifier, but have some questions for our particular needs.

Is it appropriate to assume that in a "quiescent" state, where the amplifier non-inverting input is at ground (V+ = +15 and V- = -15) and the output is connected to the inverting input, that this buffer amplifier will set the output to 0 V?  Or, is the offset relationship between what the amplifier manages at the input, and the associated output voltage, significantly unrelated?

In a quick simulation (using LT Spice, attached model) I find the output sits at +58.24uV.  

Our application presently uses a simple LM741H with an external bias trim, allowing us to set the output to 0 when our input is at a quiescent state.  The signal at the input to our buffer is from a transimpedance amplifier acting as an integrator (IVC102) with significant gain.. we are measuring femptoamps of current.  Before starting the integration, the IVC102 output does not generally sit perfectly at 0, and therefore a non-inverting buffer with offset (the LM741H) is used.  Our integrator output rate of change is anywhere between 10uV/sec and 0.5V/sec.

Is this a suitable application (and expectation) for the AD8538?  

Version 4
SHEET 1 880 680
WIRE 224 80 112 80
WIRE 448 80 304 80
WIRE 336 128 336 48
WIRE 48 144 -16 144
WIRE 112 144 112 80
WIRE 304 144 112 144
WIRE 448 160 448 80
WIRE 448 160 368 160
WIRE -16 176 -16 144
WIRE 48 176 48 144
WIRE 128 176 48 176
WIRE 256 176 208 176
WIRE 304 176 256 176
WIRE 336 272 336 192
WIRE 256 288 256 176
WIRE -16 336 -16 256
WIRE 16 336 -16 336
WIRE 96 336 16 336
WIRE -96 368 -96 304
WIRE 96 368 96 336
WIRE 16 384 16 336
WIRE -96 496 -96 448
WIRE 96 560 96 448
FLAG -96 496 0
FLAG 16 384 0
FLAG -96 304 +15
FLAG 96 560 -15
FLAG 336 48 +15
FLAG 336 272 -15
FLAG 256 288 0
SYMBOL Opamps\\AD8538 336 96 R0
SYMATTR InstName U1
SYMBOL voltage -96 352 R0
WINDOW 123 0 0 Left 0
WINDOW 39 24 124 Left 2
SYMATTR InstName V1
SYMATTR Value 15
SYMATTR SpiceLine Rser=0
SYMBOL voltage 96 352 R0
WINDOW 123 0 0 Left 0
WINDOW 39 24 124 Left 2
SYMATTR InstName V2
SYMATTR Value 15
SYMATTR SpiceLine Rser=0
SYMBOL voltage -16 160 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V3
SYMATTR Value SINE(0 14 100)
SYMBOL res 224 160 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R1
SYMATTR Value 10000
SYMBOL res 320 64 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R2
SYMATTR Value 0
TEXT -130 642 Left 2 !.tran 10m
TEXT 216 360 Left 2 ;Connect to test output offset
LINE Normal 256 240 336 352 2

  • Hi,

    First, please note that AD8538 supply should be 2.7 to 5V single supply or +/-1.35  to +/-2.5V dual supply.

    A buffer with non inverting pin to ground as you described could have some offset in the output but it could be calibrated. There could be many factors contributing to the offset error aside from the inherent offset voltage of the part you will use such as equipment, boards, wiring. AD8538 have very low offset voltage due to its auto zero architecture in the input, so this part is expected to have very minimal offset contribution in the ouput.