Precision Single-Ended to Differential... on the cheap? - Page 1
Forum section: electronics / projects

author: exmadscientist
author_rank: Frequent Contributor
timestamp: December 14, 2020, 06:20:45 am
content: I've got a couple of signals in a design that would benefit from being transmitted off-board differentially rather than single-ended. However, they are sensitive signals and must be transmitted with low offset -- the design target is 60 µV.There are plenty of standard circuits to do this, most of them involving difference amplifiers or instrumentation amplifiers. These work, but they're (a) expensive and (b) usually have largeish offsets at gain of 1. (By the way, don't trust the headline datasheet spec for offset. It is, of course, measured in the conditions most favorable to the manufacturer... and low gain is usuallyworstfor the offset spec due to how the difference amp stage works.Other things to consider:Of course, I've got not much money to do it: I can spare maybe $2 or so per channel (that's US 1k budgetary pricing).Gain of 1 would be best, though I can in some cases accommodate gain of 2. Any more than that and I'm out of headroom down the line. (Even gain of 2 may be pushing it.)Source impedances are high to very high (up to around 20 megs), which is why these signals need buffering.When I say "offset" here, I'm talking about differential offset "SIGNAL_P − SIGNAL_N", not common-mode offset. Common-mode offset doesn't matter as long as it's sane.Supply rails are, let's say, single-supply +12V. That's not quite true, but it's a decent simplification of a complex system.Input signal range is down to about 1V from ground/VEE and stays several volts away from VCC.Any ideas? I've scratched my head and come up blank, but there's plenty of people around here who have some really great ideas... I'd be keen to hear if you have any for this one.

author: Berni
author_rank: Super Contributor
timestamp: December 14, 2020, 06:38:15 am
content: 60uV offset does not sound that bad. Just use a chopper autozero opamp and its done. But when you are talking about 20Meg source impedance the more important spec on an opamp is the input bias currents.The whole idea of using differential output seams kinda strange tho. I don't see what benefit you would get from sending your signal as differential. Especially when you have multiple channels this just means extra wires. I would send the signal over as regular single ended but also run a seperate signal ground wire along with it, then use a differential input on the other end on the cable to sense voltage between the signals and the common signal ground wire.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 14, 2020, 10:13:35 am
content: I also see no real need for a conversion: just use the ground as the negative side of the differential signal. The buffer is the more tricky part with a high impedance signal. With 20 MOhms most AZ amplifiers are likely not a good choice, more like the lowest bias ones, like LTC2054 or max4238. For < 60 µV offset one could also use a good conventional FET based O, like OPA376 or OPA191 if the budget allows. It gets easier / cheaper if it is only about offset drift and a fixed offset could be compensated (e.g. digitally).

author: Non-Abelian
author_rank: Contributor
timestamp: December 14, 2020, 10:30:30 am
content: Quote from: exmadscientist on December 14, 2020, 06:20:45 amLook at the That1606/1646, Though the gain might be an issue.

author: ElectronRob
author_rank: Contributor
timestamp: December 14, 2020, 10:45:02 am
content: What frequency are these signals at?

author: David Hess
author_rank: Super Contributor
timestamp: December 15, 2020, 07:17:08 pm
content: Quote from: Kleinstein on December 14, 2020, 10:13:35 amAre there any chopper-stabilized or automatic-zero operational amplifiers suitable for high impedances? In the past I have made estimates based on the ratio of published input current noise to input voltage noise but practical circuits always required low AC impedance at both inputs to suppress charge injection.If anything, taking advantage of the lower voltage noise on newer parts makes the situation worse if you believe the datasheet specifications.

author: David Hess
author_rank: Super Contributor
timestamp: December 15, 2020, 07:20:52 pm
content: Quote from: Berni on December 14, 2020, 06:38:15 amThat is exactly what I recommend; buffer the high impedance signal and then send it along with a ground reference as a differential pair to be handled at the receiving end.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 17, 2020, 10:53:50 am
content: Quote from: David Hess on December 15, 2020, 07:17:08 pmThe choices for very high impedance are not that great. The best bets I know for this end are the LTC2054 and with a reduced (external) clock maybe the old ICL7650 and related.The data-sheet numbers for the current noise of the AZ OPs are somewhat suspicious. Some seem to be OK but others could be just a lower limit derived from the bias current. Some OPs just look to good to be true (e.g. MCP6V51 with 4 fA/Sqrt(Hz) and still 11 nV/Sqrt(Hz) or AD8628 with some 5 fA/SQRT(Hz))It can actually be quite hard to measure, as the source impedance can make a difference on the voltage noise and bias current too. It usually needs extra filtering (capacitance to ground and maybe more) at the input to avoid ugly effects from the input impedance on the offset. Even with normal OPs the current noise figures are not that reliable.