Hi!
I try to simulate a high-side current measurement circuit. But when i simulate with different temperatures the current output curve moves (due to offset) much more than I expect.
For the AD8538 the offset voltage drift is about 0.03uV/C. So when I go from 25C to 75C I expect the input offset on the OP to change with about 0.03*50 = 1.5uV. And this offset should correspond to a current of about 1.5uV / 0.05 = 30uA (R1=0.05 is the current sense resistor).
Running the simulation however results in about 300mA of current change (blue curves for 25 and 75C, green for the reference current).
So my question is what is wrong with my setup or understanding of the circuit? It seems not right the current offset should be so large?
I attach the LTSpice simulation setup below. Just remove the .txt extension in the filename (I was not able to upload the circuit directly).
Best regards
Joakim
Version 4 SHEET 1 1348 856 WIRE -272 -208 -880 -208 WIRE 208 -208 -192 -208 WIRE -880 -160 -880 -208 WIRE -880 -160 -1088 -160 WIRE -752 -160 -880 -160 WIRE -656 -160 -752 -160 WIRE -128 -160 -576 -160 WIRE 688 -160 -128 -160 WIRE 816 -160 688 -160 WIRE 688 -144 688 -160 WIRE 816 -128 816 -160 WIRE 960 -128 816 -128 WIRE -128 -112 -128 -160 WIRE -752 -80 -752 -160 WIRE 816 -64 816 -128 WIRE 832 -64 816 -64 WIRE -1088 -48 -1088 -160 WIRE 784 -48 752 -48 WIRE 960 -48 960 -128 WIRE 208 -32 208 -208 WIRE 688 -32 688 -64 WIRE 752 -32 752 -48 WIRE 752 -32 688 -32 WIRE -128 -16 -128 -32 WIRE -16 -16 -128 -16 WIRE 176 -16 64 -16 WIRE 336 0 240 0 WIRE 752 0 752 -32 WIRE 752 0 416 0 WIRE 176 16 -112 16 WIRE 832 48 832 32 WIRE 832 48 816 48 WIRE -752 80 -752 0 WIRE -656 80 -752 80 WIRE -240 80 -576 80 WIRE -112 80 -112 16 WIRE -112 80 -160 80 WIRE 816 96 816 48 WIRE 960 96 960 32 WIRE 960 96 816 96 WIRE 208 128 208 32 WIRE -752 144 -752 80 WIRE -752 144 -832 144 WIRE -832 176 -832 144 WIRE -752 176 -752 144 WIRE 816 176 816 96 WIRE 816 208 816 176 WIRE -832 288 -832 256 WIRE -752 288 -752 256 WIRE -752 288 -832 288 WIRE 480 288 480 256 WIRE 480 304 480 288 WIRE 816 304 816 288 WIRE -1088 320 -1088 32 WIRE -752 320 -752 288 WIRE -752 320 -1088 320 WIRE -752 352 -752 320 FLAG -752 352 0 FLAG 816 304 0 FLAG 816 176 Vsens FLAG 480 384 0 FLAG 480 176 0 FLAG 480 288 Imeas FLAG 208 128 0 SYMBOL res -768 -96 R0 SYMATTR InstName R1 SYMATTR Value 0.05 SYMBOL res -144 64 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R2 SYMATTR Value 5k SYMBOL res -560 -176 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R3 SYMATTR Value 200 SYMBOL res -112 -16 R180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R4 SYMATTR Value 5k SYMBOL current -752 176 R0 WINDOW 123 24 28 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName I1 SYMATTR Value PULSE(10m 2 0 10m 10m 100m 200m 1) SYMBOL voltage -1088 -64 R0 WINDOW 123 24 72 Left 2 WINDOW 39 24 138 Left 2 SYMATTR SpiceLine Rser=0.01 SYMATTR InstName V1 SYMATTR Value 5 SYMBOL res 800 192 R0 SYMATTR InstName R5 SYMATTR Value 1k SYMBOL pmos 784 32 M180 SYMATTR InstName M1 SYMATTR Value BSS84 SYMBOL res 432 -16 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R7 SYMATTR Value 1k SYMBOL res 672 -160 R0 SYMATTR InstName R8 SYMATTR Value 1Meg SYMBOL Opamps\\AD8538 208 -64 R0 SYMATTR InstName U1 SYMBOL res 944 -64 R0 SYMATTR InstName R9 SYMATTR Value 1Meg SYMBOL res -816 272 R180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R10 SYMATTR Value 1Meg SYMBOL res -560 64 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R11 SYMATTR Value 200 SYMBOL bi 480 176 R0 SYMATTR InstName B1 SYMATTR Value I={V(Vsens)*4} SYMBOL res 464 288 R0 SYMATTR InstName R12 SYMATTR Value 1 SYMBOL res -176 -224 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R13 SYMATTR Value 0.01 SYMBOL voltage 80 -16 R90 WINDOW 123 24 72 Left 2 WINDOW 39 24 138 Left 2 SYMATTR SpiceLine Rser=0.01 SYMATTR InstName V3 SYMATTR Value {Vos} TEXT -936 528 Left 2 !.tran 10m TEXT -944 608 Left 2 !.param Vos 0 TEXT -944 568 Left 2 !.temp 25 75 TEXT -704 296 Left 2 ;Linear sweep current from 10mA to 2A in 10ms TEXT 328 416 Left 2 ;Convert voltage Vsens to current
Joakim,
I don't know of any op amp from any manufacturer that models any parameters over temperature.
I looked at the AD8538 spice model and ran a sim 25 -> 70 and Vos changed by 12 millivolts;
obviously wrong. The other problem is when we write spice models, we generally use TYPs
at 25C, so the only way I know of to do a worst case error budget analysis is with a spreadsheet.
Harry