The following circuit is the front-end of a floating voltage and current measurement device intended for 230V mains.
V2 is for mains, R2 is the load, R1 is for current measurement. It simulates flawlessly, with the gain values adjusted to match the desired range.
In the real circuit MEASURED_CURRENT is -40mV (relatively to VREF) for V2=0 and BUFFERED_CURRENT is -79mV. Otherwise the circuit more or less works, with the output voltages following the gain settings, but distorted by these huge offsets.
I don't understand this, the LTC6242 is genuine (from Mouser) and the specs say it is Rail-to-Rail, low offset (125uV max). The RR part should even not be important here, as the circuit is designed to have its virtual mass at VDD/2 (~1.6V), certainly within the opamp's common mode range.
The highest gain there is 12.5, so it should result in ~1.6mV (worst case) of static error after the amplifier stage and a negligible microvolt-range distortion introduced by the followers. But I have -80mV to start from, which translates into ~600mA input current error. How can it be *that* bad?
All the measurements are performed with DC input.
Best regards, Piotr
Version 4 SHEET 1 1032 680 WIRE -1792 -720 -1792 -784 WIRE -1472 -592 -1584 -592 WIRE -1312 -592 -1392 -592 WIRE -640 -544 -672 -544 WIRE -512 -544 -560 -544 WIRE -400 -544 -432 -544 WIRE -1440 -416 -1440 -480 WIRE -1584 -400 -1584 -592 WIRE -1472 -400 -1584 -400 WIRE -80 -400 -80 -512 WIRE -1312 -384 -1312 -592 WIRE -1312 -384 -1408 -384 WIRE -1248 -384 -1248 -464 WIRE -1248 -384 -1312 -384 WIRE -1184 -384 -1248 -384 WIRE -1472 -368 -1488 -368 WIRE -528 -368 -528 -432 WIRE -672 -352 -672 -544 WIRE -560 -352 -672 -352 WIRE -400 -336 -400 -544 WIRE -400 -336 -496 -336 WIRE -320 -336 -320 -416 WIRE -320 -336 -400 -336 WIRE -288 -336 -320 -336 WIRE -1440 -320 -1440 -352 WIRE -560 -320 -576 -320 WIRE 48 -320 48 -336 WIRE -128 -304 -128 -336 WIRE -128 -304 -288 -304 WIRE 16 -304 -128 -304 WIRE 512 -304 384 -304 WIRE 224 -288 80 -288 WIRE -672 -272 -672 -352 WIRE -576 -272 -672 -272 WIRE -528 -272 -528 -304 WIRE -80 -272 -80 -320 WIRE 16 -272 -80 -272 WIRE -672 -256 -672 -272 WIRE -576 -256 -576 -272 WIRE -80 -256 -80 -272 WIRE 48 -224 48 -256 WIRE 448 -224 448 -240 WIRE 384 -208 384 -304 WIRE 416 -208 384 -208 WIRE -224 -192 -224 -224 WIRE 512 -192 512 -304 WIRE 512 -192 480 -192 WIRE 576 -192 512 -192 WIRE -288 -176 -288 -304 WIRE -256 -176 -288 -176 WIRE 416 -176 -80 -176 WIRE -128 -160 -128 -304 WIRE -128 -160 -192 -160 WIRE -928 -144 -944 -144 WIRE -800 -144 -928 -144 WIRE -672 -144 -672 -176 WIRE -672 -144 -720 -144 WIRE -576 -144 -576 -176 WIRE -576 -144 -672 -144 WIRE -400 -144 -400 -336 WIRE -256 -144 -400 -144 WIRE -80 -144 -80 -176 WIRE 448 -128 448 -160 WIRE -672 -96 -672 -144 WIRE -224 -96 -224 -128 WIRE -928 -48 -928 -144 WIRE 48 -48 48 -64 WIRE -128 -32 -128 -160 WIRE 16 -32 -128 -32 WIRE 224 -16 80 -16 WIRE -80 0 -80 -64 WIRE 16 0 -80 0 WIRE -80 32 -80 0 WIRE 48 48 48 16 WIRE -1584 112 -1584 -400 WIRE -1424 112 -1584 112 WIRE -1312 112 -1344 112 WIRE -928 112 -928 32 WIRE -928 112 -1232 112 WIRE -672 112 -672 -16 WIRE -672 112 -928 112 WIRE -80 208 -80 112 FLAG -1792 -640 0 FLAG -528 -272 0 FLAG -528 -432 VDD FLAG -1792 -784 VDD FLAG -320 -416 MEASURED_CURRENT IOPIN -320 -416 Out FLAG -1440 -320 0 FLAG -1440 -480 VDD FLAG -1248 -464 MEASURE_VOLTAGE IOPIN -1248 -464 Out FLAG 448 -128 0 FLAG 448 -240 VDD FLAG 576 -192 VREF IOPIN 576 -192 Out FLAG 48 48 0 FLAG 48 -64 VDD FLAG -80 208 0 FLAG -80 -512 VDD FLAG 48 -224 0 FLAG 48 -336 VDD FLAG -1488 -368 VREF IOPIN -1488 -368 In FLAG -576 -320 VREF IOPIN -576 -320 In FLAG -944 -144 VREF IOPIN -944 -144 In FLAG -224 -96 0 FLAG -224 -224 VDD FLAG -128 -336 BUFFERED_CURRENT SYMBOL voltage -1792 -736 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value 3.3v SYMBOL voltage -928 -64 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V2 SYMATTR Value SINE(0 10 50) SYMBOL res -704 -160 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R1 SYMATTR Value 10m SYMBOL res -688 -112 R0 SYMATTR InstName R2 SYMATTR Value 32.3 SYMBOL Opamps\\LTC6242 -528 -336 R0 SYMATTR InstName U2 SYMBOL res -688 -272 R0 SYMATTR InstName R5 SYMATTR Value 2k4 SYMBOL Opamps\\LTC6242 -1440 -384 R0 SYMATTR InstName U3 SYMBOL res -1376 -608 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R7 SYMATTR Value 7k5 SYMBOL Opamps\\LTC6242 448 -192 R0 SYMATTR InstName U4 SYMBOL res -1328 96 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R11 SYMATTR Value 1.2Meg SYMBOL Comparators\\LTC6752 48 -80 R0 SYMATTR InstName U1 SYMBOL res -96 -416 R0 SYMATTR InstName R8 SYMATTR Value 2k4 SYMBOL res -96 16 R0 SYMATTR InstName R12 SYMATTR Value 2k4 SYMBOL Comparators\\LTC6752 48 -352 R0 SYMATTR InstName U5 SYMBOL res -96 -160 R0 SYMATTR InstName R13 SYMATTR Value 7k5 SYMBOL res -96 -272 R0 SYMATTR InstName R14 SYMATTR Value 7k5 SYMBOL res -592 -272 R0 SYMATTR InstName R9 SYMATTR Value 2k4 SYMBOL res -544 -560 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R6 SYMATTR Value 7k5 SYMBOL res -416 -560 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R10 SYMATTR Value 7k5 SYMBOL res -1216 96 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R3 SYMATTR Value 1.2Meg SYMBOL Opamps\\LTC6242 -224 -160 R0 SYMATTR InstName U6 TEXT -1824 504 Left 2 !.tran 2e-2