On a single piece of silicon on which there is more then one op amp, are the input offset voltages at all correlated, i.e. would they be expected to be same direction and similar magnitude?
\$\begingroup\$
\$\endgroup\$
3
-
\$\begingroup\$ There may be some similarity, but not reliable enough to be useful. \$\endgroup\$– user16324Commented Mar 5, 2018 at 22:23
-
\$\begingroup\$ If you need low offset then a laser trimmed chip is better for you. digikey.bg/product-detail/en/texas-instruments/THS4551IDGKT/… \$\endgroup\$– D.A.S.Commented Mar 5, 2018 at 23:49
-
\$\begingroup\$ What will happen is thermal crosstalk between the OpAmps. The deltaVoffset/deltaTemperature spec is a DC spec; you'll need to test the actual circuit with the actual waveforms, and measure distortion of the 2 or of the 4 opams. \$\endgroup\$– analogsystemsrfCommented Mar 6, 2018 at 2:49
Add a comment
|
5 Answers
\$\begingroup\$
\$\endgroup\$
1
No, you can't assume anything about correlation between opamps on the same chip unless the datasheet explicitly says so. I don't remember ever seeing a datasheet say anything about offset voltages of opamps on a chip relative to each other.
Think about it: offset voltages are due to the slight mismatches between transistors in a chip. Some transistor parameters are random, but others may correlate with where they are on the wafer. However, the input transistors of an opamp are already near each other, and likely much closer to each other than the input transistors of other opamps on the same chip. This doesn't leave much mechanism for the offset voltages between nearby opamps to be correlated somehow.
answered Mar 5, 2018 at 22:26
-
\$\begingroup\$ I agree. I don’t have the numbers in front of me but I just tested an OPA4388IDR from TI and all four opamps had different offset voltages. \$\endgroup\$– mikeCommented Jun 7, 2022 at 19:36
\$\begingroup\$
\$\endgroup\$
2
No.
The offset voltage comes from the difference between the two input transistors in the same opamp.
The input transistors in this TL072 are interleaved so that they have the same center, so that if the transistor parameters are varying linearly across the die, the average parameters are the same. Despite this the transistors are still slightly different because the variation is not exactly linear.
So if the two transistors right next to each other are mismatched, why would the ones on the other half of the die have the same mismatch?
-
\$\begingroup\$
+1I really like this answer - an IC that looks just like your avatar! Seriously, this is a good answer because it includes some helpful insight into the why. I've never heard of interleaved transistors. I see identical looking patterns, but nothing I could pick out as two transistors with actually interleaved structures. Is it possible to describe where that's shown, or a link to a description? \$\endgroup\$– uhohCommented Mar 6, 2018 at 12:42 -
1\$\begingroup\$ The input transistors are the large structures at the bottom. Left and right are the two opamps. \$\endgroup\$– τεκCommented Mar 6, 2018 at 13:01
\$\begingroup\$
\$\endgroup\$
For absolute values-NO. For drift over time, they will have there independent offsets and gain tracking issues as well.
For dual and quad op-amps, precision means independent gain and offset for each channel.
With independent gain and offset, there should be some correlation over time, but no datasheet would ever state that. Reason is that the user ambient temperature and voltage and loading of outputs is unknown.
If one of four channels has a heavy load, any hint of correlation is gone.
\$\begingroup\$
\$\endgroup\$
No guaranteed correlation assured. Best you can expect is that they should be within data sheet specs as long as you correctly apply the part within rated operational ranges.
answered Mar 5, 2018 at 22:25
\$\begingroup\$
\$\endgroup\$
This is not an opamp, but it is a differential pair, sold as having very good thermal tracking between the two transistors.
It is actually made on die as thousands of individual transistors, that are then connected checkerboard fashion in parallel into two interleaved 'super-match' transistors.
I came across this device years ago not because we needed superb matching between the two transistors, but because we needed a very low noise, low impedance, transistor for low frequency 50 Ω amplification duty (post mixer in a phase noise test set), and simply used the two transistors in parallel.
