"Virtual Ground" / Buffered Ground Reference Stability - Page 1
Forum section: electronics / projects

author: ignilux
author_rank: Unknown
timestamp: December 30, 2020, 03:10:51 pm
content: Hi, all-I'm working on a design to measure small currents (hundreds of pA to uA) with a zero drift amplifier (LTC2055), and am doing so on a single 5V rail. Because of this and the desire to place my signals in the center of the amplifier's common mode range, I'm buffering the half rail voltage through an AD8607 to provide a "virtual ground" reference. That's all fine and good, but between the reed relays to switch ranges and the rotary switch for range selection, my PCB is 75 x 100mm. I've decided that the virtual ground should have local decoupling due to the size of the board, so I simulated the AD8607's ability to drive multiple, distributed, capacitive loads in a non-inverting configuration. Between a series resistor to isolate the opamp from the bypass caps, dividing down the input and boosting the gain of the circuit, and adding a compensation cap in parallel with the feedback resistor it looks for all the world like I have a pretty stable amplifier.However, I'd like your thoughts and insights with respect to my LTspice simulations below. The first of which shows the step response of the amplifier. I'm not concerned with an overly snappy transient response, since this should ideally be a pure DC output. The other simulation shows frequency domain stability information. There appears to be plenty of phase margin, and the magnitude of the gain crosses unity at -20dB / decade. The output network was obviously just a (very) rough estimate of trace inductance and distributed decoupling, but I thought it would serve as a decent "worst case" scenario.Anyway, any and all thoughts are welcome and appreciated.Cheers!

author: David Hess
author_rank: Super Contributor
timestamp: December 31, 2020, 03:56:04 am
content: Swamping the output of the operational amplifier with a small bulk decoupling capacitor like an aluminum electrolytic or tantalum can work while providing low AC output impedance even if the operational amplifier is not specified to drive large capacitive loads. The reason this works is that a large enough capacitance implements dominant pole frequency compensation. The large ESR of the capacitor adds a zero to the response which aids stability so do not try this with a ceramic capacitor.Some operational amplifiers are specified to drive any capacitive load.

author: ignilux
author_rank: Unknown
timestamp: December 31, 2020, 01:32:55 pm
content: David-Thanks for your response. I was originally planning to use 1u, X7R ceramics, but it sounds like that's not the best idea. I'm assuming I can stick with ceramics if I provide a small resistance in series with each cap? Something around 2R2? I'm concerned that this might defeat the purpose of local decoupling, though. While it's certainly simpler, is there reason to believe that this method of compensation would be more reliable / less immune to variations in parastics, etc than the circuit that I presented?Re: opamps that drive any capacitive load, the primary reasons for choosing the AD8607 are low power (this will be a battery operated device), and I already have some on hand.Thanks again!

author: exe
author_rank: Unknown
timestamp: December 31, 2020, 02:07:46 pm
content: There ishttps://www.ti.com/product/TLE2426which is designed to split the rail, and specified for capacitive load. May be it will be easier to use it.

author: David Hess
author_rank: Super Contributor
timestamp: December 31, 2020, 03:50:33 pm
content: Quote from: ignilux on December 31, 2020, 01:32:55 pmYes, adding a series resistor solves the problem with low ESR.QuoteIn my experience it gives the best results. The bulk decoupling capacitor at the output also stabilizes things despite the ceramic local decoupling capacitors.The alternative is shown as the last example here which discusses how to stably drive a capacitive load with any operational amplifier:https://www.analog.com/en/analog-dialogue/articles/ask-the-applications-engineer-25.htmlQuoteLow power makes things more difficult because low power parts are less tolerant of capacitive loads.Linear Technology (Analog Devices) makes some operational amplifiers which can drive any capacitive load, and some of them are low power like the LTC6258 and LTC6255 and for higher voltages, the LT1351.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 31, 2020, 05:07:31 pm
content: The resistor (68 Ohms) at the OPs output isolates it from the capacitive load. So this should be OK, if there is not too much load current to the virtual ground. A good circuit should be able to tolerate the virtual ground to move a round a little, if no too fast.The actual measurement circuit however looks odd. The LTC2055 is a low bias AZ OP, but it may still not be very happy with 10 M source impedance. With such a low current, I would look at a non AZ OP and using a TIA configuration with an even larger resistor. If AZ is needed, maybe look at the OPA380 - it is specially made for an AZ TIA.

author: ignilux
author_rank: Unknown
timestamp: December 31, 2020, 06:32:57 pm
content: Quote from: David Hess on December 31, 2020, 03:50:33 pmDavid-Thanks again for the informative reply. I've seen that article in the past, and it is certainly helpful. Playing around in LTspice with the various compensation methods was yielding results that weren't straightforward, though. At least with the AD8607. I had looked through Analog's portfolio of "C-load" amplifiers in the past couple of days, but I didn't see the LTC6255. That looks perfect for my application.Quote from: Kleinstein on December 31, 2020, 05:07:31 pmKleinstein-Thanks for your input. Indeed, at the worst case bias current of +/- 150pA over the full temperature range, that results in a 1.5mV error. I noticed this at some point earlier on, but it was before I added the 10M range. It is certainly pointless to use an auto zero amplifier if the bias current causes an error three orders of magnitude greater. As far as a transimpedance configuration goes, that was certainly something that I toyed with. Specifically, I like the the way that the Keithley 480's TIA stage is built (see service manual capture below). The matched JFET front end won't be cheap, but I figure something like an LSK389 should do nicely, or I might grade some CPH6904 that I have around here somewhere. The JFETs also relaxe requirements on the opamp, since they're manually trimming the offsets. Or, at least Iassumethat's what they're doing with R115/R116. According to the service manual U104 is an LF356.

author: ignilux
author_rank: Unknown
timestamp: December 31, 2020, 06:37:17 pm
content: Whoops, forgot the Keithley TIA:Also, to exe-As far as I know the TLE242x parts are simply an integrated version of what I'm trying to do. The downside is that they don't give the user access to the underlying circuitry.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 31, 2020, 07:59:33 pm
content: There is essentially no more need to use discrete JFETs for a TIA. Ready made OPs, like OPA145 are pretty low noise and still low bias not so easy to beat with discrete FETs.If AZ is needed one could build a circuit similar to the OPA380 internals - with larger caps and a lower BW OP. Depending on the needs, one could also use a precision type without AZ. Noise wise a TIA has some definite advantages over shunt and amplification with a larger resistor (e.g. 10 x) the demands on the offset can be smaller than with the shunt circuit and just a FET OP can be sufficient. The main downside is that a short would be seen as high current and higher current needs more power.The difficulty of high resistance in combination with the LTC2055 is not just because of the bias, but also because of strange effect that can happen when the input impedance changes. So one may need some extra filtering to isolate the AZ OP from the input at higher frequencies. The required caps can slow down the low current range.Unless you need it fast, the LTC2055 can do a gain of 100 in one stage. If it needs to be fast, better use a 2nd OP inside he loop, as a compound amplifier. So what is the intended speed ?

author: ignilux
author_rank: Unknown
timestamp: January 01, 2021, 02:20:37 pm
content: Quote from: Kleinstein on December 31, 2020, 07:59:33 pmAll good points. The OPA145 has very low voltage noise, but the maximums for input bias current, offset current, and offset voltage over temperature give me pause. LTC6081 has an order of magnitude lower maximum figures in all three categories, though it doesn't match the noise performance.The LTC2055 datasheet goes into this a bit, and how the auto zero sampling circuitry can affect bias current. They recommend a cap in parallel with the feedback resistor, which of course slows down the amplifier.Yeah, I'm not sure why I did two stages of Av=10. Force of habit, I guess. I want to have some decent bandwidth for watching startup behavior of low power circuits, etc, but this will largely be a near-DC measurement device. I'd say 1 kHz or so would be plenty for me. LTC6081 has a GBW of 3.6 MHz, so using one channel for the TIA and one for an inverting 100x gain stage should leave plenty of bandwidth.Reading the theory of operation in the Keithley 480 service manual reveals a clever trick: the 1n, 100n, and 10u ranges switch a 10x attenuation in series with the feedback network. This results in 10x voltage gain for those ranges, so they only need to use 4 precision resistors for the 7 ranges. I don't think I'll go to the trouble on my end, but it's neat to see things like this.