Instrumentation amps: what causes gain ripple/overshoot at unity gain? - Page 1
Forum section: electronics / projects

author: hanakp
author_rank: Regular Contributor
timestamp: September 19, 2022, 03:41:39 pm
content: I worked with instrumentation amplifiers lately and I noticed that with some types, the gain vs. frequency plot doesn't monotonically fall off. There is a small ripple or overshoot, usually it happens only at unity gain. Here is how it typically looks, it's from AD8221 datasheet:Edit: hmm, the inline attachments don't work anymore?Normally, the ripples are small, about 1 dB. But what causes it? Some parasitics inside the IC? Why some amps have these ripples and others don't? I checked several instrumentation amp guides, but they don't explain it. I'm asking because I noticed that with some ICs, the source impedance has marked effect on these ripples. With low source impedance, the ripples are always like in the datasheet.Now I have a sensor that acts as 200:1 compensated RC divider with 6M and 30k resistors (and appropriate capacitors). Both amp inputs are connected to these sensors. With AD8221, that causes the ripple to increase to about 2 dB. But with (higher bandwidth) AD8422, it increases to 8 dB! Why? Is there some way to suppress this effect? Or can I at least predict/compute how bad it will become from some parameters in the datasheet?(I know what you're thinking: "just use an amp that doesn't have these ripples". Well, I'd love to, but many of them are sold out. Plus, I'm hoping to learn what causes the ripples in the first place.)

author: tggzzz
author_rank: Super Contributor
timestamp: September 19, 2022, 04:00:16 pm
content: To cut a long story short, such overshoot in the time domain or peaks in the frequency domain (they are duals) is caused by the combination of the amplifiers internal circuits in conjunction with the external feedback networks. Hence to remove them, either change the amplifier's internal circuits (not always possible!) or the external feedback circuits. The latter implies reducing the overall bandwidth.Since this is an instrumentation amp, it may be necessary to closely match several of the external components in order not to reduce the CMRR and other parameters.I would suggest that you carefully understandallof the instrumentation amps's specifications (e.g. CMRR as a function of frequency), and compare those with the minimum necessary for your application. That will indicate what options are possible in your case.Consider using an analogue simulator to model your circuit and/or application, but beware that off-the-shelf models may not include effects that are important to your application.

author: magic
author_rank: Super Contributor
timestamp: September 19, 2022, 04:43:55 pm
content: See "theory of operation".At unity gain there is no external feedback network, just two unity gain buffers consisting of Qn,An,Cn,Rn each, driving the second stage difference amplifier A3. If you see gain peaking at unity gain only, it's gain peaking in those buffers, because the difference amp doesn't care about 1st stage gain.Gain peaking occurs because the buffers have insufficient phase margin. This could be improved by increasing Rn or Cn, just not by you. The same poor phase response which causes peaking at unity gain provides useful bandwidth extension at small positive gains, so zero peaking may not always be desirable commercially.The input impedance dependence has to be an effect of unexpected AC currents flowing out of the input pins, correlated in phase with your signal. Reminds me of this thread.https://www.eevblog.com/forum/beginners/jfet-frontend-amplifier-stability/Quote from: hanakp on September 19, 2022, 03:41:39 pmPerhaps not a bad idea.Alternatively, dunno, maybe there are bare difference amplifier chips that omit all that "instrumentation amp" input stage stuff, which could be driven by a pair of external buffers tweaked to deliver good performance at unity gain with your particular source impedance.Noise can be an issue - this circuit has only two transistors in the differential signal path, but two unity gain opamps, for example, have four.

author: David Hess
author_rank: Super Contributor
timestamp: September 19, 2022, 08:24:50 pm
content: Quote from: magic on September 19, 2022, 04:43:55 pmI agree with magic. Many feedback amplifiers display that behavior. At unity gain, the closed loop bandwidth is highest, and the lack of phase margin has the greatest effect on the response, and this can be seen in the overshoot in the small signal response at low gain which is shown in figure 37 in the datasheet. As the gain is increased, shown in figures 38, 39, and 40, the overshoot decreases and then vanishes completely.For current feedback amplifiers, it can be corrected by increasing the feedback resistance limiting bandwidth. For the AD8221, make sure parasitic capacitance at pins 2 and 3 is minimized. If applicable, cut the ground plane away from the gain pins.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 19, 2022, 08:44:15 pm
content: Most instrumentation amplifiers are made with focus on performance with at least some gain. The gain 1 one case is relatively rare and they may compromise some peaking / ringing for better GBW in the modes with gain.

author: magic
author_rank: Super Contributor
timestamp: September 20, 2022, 06:50:26 am
content: Somebody posted and deleteda useful suggestion: run the chip at a gain, then divide the output.+ no gain peaking if enough gain+ potentially better noise performance (the second stage often sucks noise-wise in those chips)- not exactly unity gain, sensitive to resistor tolerance- less bandwidth if too much gain- risk of clipping

author: mikerj
author_rank: Super Contributor
timestamp: September 20, 2022, 08:13:38 am
content: Quote from: magic on September 20, 2022, 06:50:26 am- significantly higher output impedance

author: jonpaul
author_rank: Super Contributor
timestamp: September 20, 2022, 08:19:34 am
content: Unity gain has the worst transient as the feedback is 100%many op amps and instr amps are not rated for unity gain.Run a bode plot and check poles/zeros.See the classic control system theory texts.Jon

author: hanakp
author_rank: Regular Contributor
timestamp: September 20, 2022, 10:00:31 am
content: Quote from: magic on September 19, 2022, 04:43:55 pmMany thanks, I think you're right on the money here. Both AD8221 and AD8422 have identical internal topology, but in AD8422, the R1 and R2 resistors have smaller value (9.9k). Since AD8422 has higher bandwidth, this implies they had to increase transistors' bias currents to fight internal parasitic capacitances. If the transistors have similar parameters in both ICs, that also translates to higher input bias currents. This is actually the case in Spice simulation, AD8422 shows ~10 times higher current which increases with frequency. But as tggzzz mentioned, I wouldn't rely on it much, because the same AD8422 model is completely missing the gain overshoot. And conversely, AD8221 model doesn't show the input current frequency dependence at all, even though on paper, they have the same input capacitance.Oh well, I'll just have to try amps that don't "suffer" from those ripples. I can't use higher gain unfortunately, because I can't easily change the sensors' voltage dividing ratio and I don't want to increase supply voltage (+-5V now). Plus, I tried setting the gain to 4, but it didn't help much. Theoretically, I could counteract the ripples with a large series resistor to the input. But I don't like it because I have no control over the input capacitance and gain accuracy would suffer due to voltage drop on the resistor.