Cascaded ADA4522 opamps rail to rail performance - Page 1
Forum section: electronics / beginners

author: raff5184
author_rank: Regular Contributor
timestamp: October 08, 2020, 02:02:23 am
content: Hi,I want to amplify a 3Vpp 500kHz signal. At the moment I'm just testing the opamp. I'm using 2 cascaded ADA4522 powered rail to rail (+-25V) and each in non inverting configuration and a gain of 4, I want to measure how much they can amplify. I'm not using any load at the moment.I can make it work only up to few tents of kHz and for low input voltage (mV p2p). After that the output is almost a DC signal.I'm attaching the first stage, which is not working. With a GBWP of 2.7MHz at 500kHz I should be able to amplify 4x, or at least something. But it's not working*C3=22uF

author: raff5184
author_rank: Regular Contributor
timestamp: October 08, 2020, 02:22:11 am
content: I'm also attaching these pictures to show how the output (green) of a single stage changes with frequency and input voltage. It is fine till 1Vpp and 40kHz

author: magic
author_rank: Super Contributor
timestamp: October 08, 2020, 05:31:02 am
content: Those waveforms seem to be limited by slew rate. There is a well known formula for how much slew rate you need to get X volts sinewave at Y Hz without distortion which I always keep forgetting, go look it up. Alternatively, many datasheets provide plots of maximum possible sine amplitude vs frequency.Something like NE5534 without compensation or a modern fancy high speed opamp would do it.

author: newbrain
author_rank: Super Contributor
timestamp: October 08, 2020, 07:06:32 am
content: Quote from: magic on October 08, 2020, 05:31:02 amYes.The ADA4522, with 55 V supply (close to the 50 V in the OP) has a positive going SR of 1.7 V/µs and 0.8 V/µs negative going.A sinusoid has a maximum slope equal to its first derivative when it crosses the 0:d/dt A sin( ω t ) = A ω cos ( ω t )for t=0 s, simply:SRmax= A ωFor ω = 2 π f = 6.28 × 500e3 Hz, and A = 2 V (as we expect 4 Vpp) we have a needed slew rate of at least 6.28 V/µs which largely exceeds the guaranteed performance.Also, look at Figure 68, page 20 of the datasheet which gives the maximum output swing vs. frequency for various supply values

author: S. Petrukhin
author_rank: Super Contributor
timestamp: October 08, 2020, 07:20:48 am
content: You have introduced the frequency dependence of the feedback with a sufficiently large capacitor C3. Remove it and the distortion will disappear.

author: magic
author_rank: Super Contributor
timestamp: October 08, 2020, 07:36:40 am
content: C3 only affects low frequencies. Such capacitor is commonly used in audio amps to stop them from amplifying their own DC offset. It makes R1 effectively become open circuit at DC and hence unity gain. At HF its impedance is next to nothing.

author: eblc1388
author_rank: Frequent Contributor
timestamp: October 08, 2020, 07:54:06 am
content: Quote from: newbrain on October 08, 2020, 07:06:32 amWith the desire gain of 4 and input Vpp=3V 500KHz, the output would be 12Vpp. Therefore the required slew rate for the first stage should be 2xpix500Kx6=18.85V/us, isn't it? The ADA4522 has two different slew rate for positive/negative going output so one should use the lower figure in design.For the second stage with a gain of 4, the op amp slew rate requirement is four times higher at 75.4V/us.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: October 08, 2020, 08:05:21 am
content: Quote from: magic on October 08, 2020, 07:36:40 amYes, I completely agree, I didn't pay attention to the frequencies.

author: newbrain
author_rank: Super Contributor
timestamp: October 08, 2020, 09:25:29 am
content: Quote from: eblc1388 on October 08, 2020, 07:54:06 amI was referring to the posted pictures where 1 Vpp input is used, but the yes it only gets worse...

author: raff5184
author_rank: Regular Contributor
timestamp: October 08, 2020, 06:26:13 pm
content: Thanks for the replies.So, if my calculations are correct, with a single opamp, if I want to amplify rail-to-rail, which is 54.6V (and not 55V, see output characteristics on page 6)1) the minimum SR needed at 500kHz is 171V/us2) The max supported frequency is 2.3kHz (at SR=0.8V/us)3) The max Vin supported (at SR=0.8V/us and 500kHz) is 255mV, but I won't be able to amplify rail-to-rail because I would need a gain of G=214. Instead, the best it can do is G=2.7MHz/500kHz= 5.4, Which is in line with the 15dB at 500kHz of Fig 32 page 14.Do these look correct?How do I find the max amplification supported with an input signal of 3Vpp and 500kHz? Is it even in range?So, cascading them is essentially pointless because the first stage shouldn't give a Vout larger than 255mV, else the second stage can't amplify properly?Quote from: magic on October 08, 2020, 07:36:40 amCorrect. Actually my value is off, I should make it larger, with 22uF the fc = 700kHz.

author: magic
author_rank: Super Contributor
timestamp: October 08, 2020, 07:10:46 pm
content: It's not about input but output. The output cannot move fast enough to produce high amplitude and high frequency waveform.This chip can only output maybe a volt or less at 500kHz, no matter what input it gets.

author: newbrain
author_rank: Super Contributor
timestamp: October 08, 2020, 08:06:56 pm
content: I'll quote magic:Quote from: magic on October 08, 2020, 07:10:46 pmAnd myself:Quote from: newbrain on October 08, 2020, 07:06:32 amFrom the picture you can see that no matter what supply, you are bound to about 1 V of output swing after 200 kHz.This is a chopper amplifier, but I think the explanation is the same as for classic ones. With a number of simplifications:in this slew rate limited situations, the overall behaviour is very similar to an integrator where the current on the tail of the input stage is slowly charging the dominant pole compensation capacitor, multiplied by the Miller effect.Feedback is of little consequence, as the input pair is completely unbalanced and acting non-linearly.

author: Vovk_Z
author_rank: Super Contributor
timestamp: October 08, 2020, 09:02:28 pm
content: If you need to amplify 3Vpp 500 kHz signal 4-10 times you may use LM6172. It is fast, cheap and wide spread. It is +-15V but that is ok for 3V*4.