ultrasonic receiver op amp - Page 1
Forum section: electronics / projects

author: diegop
author_rank: Contributor
timestamp: July 14, 2016, 09:40:15 pm
content: Hello everyone. I am working on an ultrasonic receiver and need some help choosing the op amp to amplify the signal. I am basing my design on this schematichttp://uglyduck.ath.cx/HC-SR04E/HC-SR04E.svgzIn that design they use a LM324, but I was wondering if there is something better that I could use with better performance or the LM324 will suffice? Would a JFET op amp be better?

author: Seekonk
author_rank: Super Contributor
timestamp: July 14, 2016, 10:13:30 pm
content: I love that input circuit. That was a stumper on another board , fellow couldn't figure out why he gain was less than expected. It has a magical quality. Hey, what frequency do you call ultrasonic and are you looking for wide band response?

author: Audioguru
author_rank: Super Contributor
timestamp: July 14, 2016, 10:21:12 pm
content: An LM324 was designed for low power so its high frequency response is poor, it is noisy and it produces crossover distortion. This circuit is simple with low gain so it might work fine but you forgot to say the frequency, I am too lazy to calculate the frequency of its multiple feedback bandpass filter.

author: diegop
author_rank: Contributor
timestamp: July 14, 2016, 10:24:53 pm
content: The frequency would be 40kHz. I am looking for a narrow bandpass filter.I was looking at other options. The LMV722http://www.mouser.com/ds/2/405/lmv722-556077.pdfseems like a much better op amp and instead of the 3 stages just 2 stages. One for the multiple feedback filter and the other for the inverting amplifier, but with more gain, say around 70 which the lmv722 should be able to do. It has a GBW of 10MHz.I also looked at the TLV2316 which has the same GBW, but it is a CMOS instead of bipolar. I am not sure which architecture would work better for this application.

author: diegop
author_rank: Contributor
timestamp: July 15, 2016, 06:03:25 pm
content: I ran some simulations for the original schematic with the LM324 and a modified one with the LMV722. In the modified schematic the filter has a Q=10 and a gain of 5.I am attaching the bode plot and the schematic. The green line is with the LM324 and the red with the LMV722. Also according to the simluation the LMV722 has a signal to noise ratio at 40kHz of 78dB and the LM324 71dB.Looking at the bode plot would the lm324 work fine or would it be better to use the lmv722?

author: Kleinstein
author_rank: Super Contributor
timestamp: July 15, 2016, 08:02:21 pm
content: A 7 dB difference in noise level is still quite a lot. Its something like twice the distance. From the DS I would even expect a larger possible difference. The inverting first stage is not that good, and the 10 K resistor at the input might give more noise than the LMV722. So with a different type of input stage even lower noise might be possible with the LM722.The cross over distortions can have a negative effect like a limited slew rate or even worse, so extra provisions might be needed to keep the output stages in the class A mode. This can negate the advantage of low current consumption. The simulation might not include this, especially not the AC simulation.The type of OP CMOS / BJT / JFET is usually not that critical, unless the source is really high impedance (e.g. way less than 1 nF).There is still the option to use a discrete BJT for the first amplification - linearity and stability is often less of an issue and one could even use it to get an adjustable amplification. A 2N3904 beats the OPs hand down, when it comes to S/N.

author: diegop
author_rank: Contributor
timestamp: July 16, 2016, 12:16:18 am
content: What changes would you recommend to improve the input stage of the LMV722?

author: Audioguru
author_rank: Super Contributor
timestamp: July 16, 2016, 02:15:08 am
content: Your sim has an extremely wide range of dB (300db!) and frequencies from almost DC to almost lightwaves. I resized it so you can see how wide it really is.A simulation uses "typical"spec's and the minimums for a lousy old LM324 opamp that you might actually have are much worse.

author: StillTrying
author_rank: Super Contributor
timestamp: July 16, 2016, 02:51:29 am
content: Quote from: diegop on July 16, 2016, 12:16:18 amI've only looked at the input stages, here's what I'd try - not saying they're improvements!Make the 1st stage non-inverting and a very much higher gain than the 2nd stage, so that the 2nd stage doesn't just amplify the noise from the first.Center both stages' frequency response on 40Khz, so both together almost give you the filter for free.If the TX is only run off 5 or 6V, drive it with 2 anti-phase waveforms to increase the output.The LMV722 looks fine to me, but if the detection stage loads the 2nd stage too much it might struggle to do 2Vp-p at 40Khz.

author: diegop
author_rank: Contributor
timestamp: July 16, 2016, 04:58:08 am
content: Thanks for all the help so far. What do you all think of 2 multiple feedback bandpass filters? I am trying to keep the design down to only 2 op amps to only use one chip, since the LMV722 only has 2 channels.I am attaching the amplitude graph and the schematic of the new design compared to the old one.At about 20 kHz and 80 kHz the amplitude reaches 0 dB. To me it seems like a good improvement over using the inverter + the filter, but I don't know if there are drawbacks that cannot be seen in the ac analysis.

author: Audioguru
author_rank: Super Contributor
timestamp: July 16, 2016, 11:42:57 am
content: Two filters in series produce a sharper response but is it good enough?

author: Kleinstein
author_rank: Super Contributor
timestamp: July 16, 2016, 11:47:07 am
content: The typical US receivers have a resonance at about 40 kHz, where they should receive the signal. Here the source impedance can be quite high (e.g. 10 K range), and loading the sensor too much will dampen the resonance, thus through away the internal filtering. So the inverting circuit is not good if narrow band reception is needed.I would make the first stage non inverting with a limited bandwidth (e.g. 20 kHz - 80 kHz; thus very low Q and still DC gain of 1) instead of an inverting filter. The second stage can than do more filtering.

author: diegop
author_rank: Contributor
timestamp: July 16, 2016, 04:33:00 pm
content: Quote from: Kleinstein on July 16, 2016, 11:47:07 amI set an impedance on the source of 10k and the gain dropped from 52 dB to 37 dB, a significant drop. By any chance do you have a link for a configuration to do what you are recommending for the first stage?

author: StillTrying
author_rank: Super Contributor
timestamp: July 16, 2016, 05:11:21 pm
content: Quote from: diegop on July 16, 2016, 04:33:00 pmI hope you used 2 separate 10K's, 1 to each input, and didn't just leave both inputs shorted.I was going to suggest improve the input model because the 2 circuit versions have very different input impedances. To get a good comparison of the gains and frequency responses I don't think you have any choice but to make 2 40Khz transducer models. There's a model here next to 'Response function'.http://bertrik.sikken.nl/bat/detuning.htmSurely you know what the non-inverting looks like, it's the last one on this page.http://www.radio-electronics.com/info/circuits/opamp_non_inverting/op_amp_non-inverting.php

author: diegop
author_rank: Contributor
timestamp: July 16, 2016, 05:30:21 pm
content: Quote from: StillTrying on July 16, 2016, 05:11:21 pmYeah, I made that mistake. I corrected that mistake and the gain went up to 38 dB instead of the 37 dB I had mentioned earlier. This is for the design with 2 multiple feedback filters.Thanks for the link to the transducer model. Yes, I know how the non-inverting configuration looks like, but I thought he was talking about a non-inverting bandpass filter. I found this document which has that configuration. I found out that it's a mix of the low pass and high pass filter configuration.http://www.physics.unlv.edu/~bill/PHYS483/op_amp_filt.pdf

author: Kleinstein
author_rank: Super Contributor
timestamp: July 16, 2016, 05:49:05 pm
content: The transducer model might be important to see the effect not only on amplitude and noise, but also on bandwidth. Depending on what you want to do with the US signal, a low bandwidth might be good or not that good. A high low bandwidth of the transducer tends to reduce the noise but it also takes longer to react to changing amplitude. So time measurements might get more tricky, though the S/N can get better.How much filtering is needed also depends on how you look at the signal - less filtering gives less artifacts / delay from the filters.

author: diegop
author_rank: Contributor
timestamp: July 16, 2016, 06:38:25 pm
content: Quote from: Kleinstein on July 16, 2016, 05:49:05 pmWhat I am going to do is measure distance. Send a few pulses with an ultrasonic transmitter and measure the time it takes for the receiver to detect the echo.

author: diegop
author_rank: Contributor
timestamp: July 16, 2016, 07:15:58 pm
content: Using the model from the link, this is how the amplitudes look. I also ran a transient simulation and the inverting + filter seems to have a slightly faster response, but I do not think it is significant.I am also going to simulate using a non-inverting configuration for the first stage