Cascaded Op-Amp Saturation Problem for a Ultrasonic Transceiver Circuit - Page 1
Forum section: electronics / projects

author: hunter84
author_rank: Newbie
timestamp: October 02, 2014, 01:18:33 pm
content: I am just trying to sense the output of an ultrasonic transceiver. My driving frequency is 40 kHz and as the amplitude of the received signal is in the range of mV's I need to use a two stage cascaded amplifier circuit. This will be a range detection application so as you know I need to measure time of flight between transmitted signal and received signal.You can find the schematic in the attachment. As you see the circuit consists of two differential amplifier stages. The Op-Amp is OPA2340 from Texas Instruments. Vcc voltage is 3.3V for the op-amp. I have also attached oscilloscope screen views. I need some comments about these.Point A is the output of first amplifier. As you see in green A signal, first part is the transmit part with some additional ringing. Then there is a small part nearly 4msec behind the transmit signal. Good, this is my received signal. First stage amplifies well and received signal has an amplitude of nearly 500mV.Strange thing starts here: B point is the output of the second stage. At instant 1, as you can see yellow B signal contains only the transmit part. Where is the received part? Why does it not amplify the received one?Then for B signal at instant 2, a small received signal occurs. At instant 3, 4 and 5 received part starts to surround the whole timebase and finally output of second stage(B signal) completely saturates and is equal to 3.3V positive rail.The time elapsed between each instant is in the order of seconds. Let me note that, amplifier stage 1 Works correctly at all instants it does not change.When I deenergize the circuit and then energize it again, the same story repeats with exactly the same order.This seems strange, can it be a noise issue? Or some problem with the layout? Maybe a dual op-amp cause that problem? Two single op-amps will be better?I am missing a point for nearly two days, could you have some comments?

author: Precipice
author_rank: Frequent Contributor
timestamp: October 02, 2014, 01:44:09 pm
content: High levels of DC amplification? Outputs sitting at supply rails? Yep. Time to AC couple before, and inbetween, stages. There's nothing you want accurate DC amplification for here.Aargh! I hit 'reply' rather than 'save'.You also seem to be using a single supply rail, with opamp ground as your ground for filtering and gain. That's going to produce weird rectification effects. Search for 'virtual ground' to make your stages behave properly.Also, that's a lot of breakthrough between transmit and receive. You should be able to reduce that massively by mechanically separating the transducers, and mounting them in rubber grommets or something else squishy, which will help if you want to detect things nearby.

author: hunter84
author_rank: Newbie
timestamp: October 02, 2014, 02:26:22 pm
content: Thank you very much for your valuable comment.For AC coupling part, you are right. Actually I am dealing with the AC signal received from the transceiver.Actually I could not understand what you mean with the virtual ground, but I will also try to disconnect op-amp ground from the main ground and directly connect it to the start point at the GND pin of 3.3V SMPS output capacitor. Do you mean sth like that?Actually I am using a transceiver mounted in a rubber grommet. I have obtained it from a car parking sensor kit. Seperate transmitters and receivers is much more simpler as you have mentioned. I have tried them and there is no problem.But at that work, I am especially concentrated on single transceiver.

author: Precipice
author_rank: Frequent Contributor
timestamp: October 02, 2014, 02:43:53 pm
content: Quote from: hunter84 on October 02, 2014, 02:26:22 pmOK, AC couple at the input to the second opamp, that'll probably fix many of your problems, for the cost of a (say) 10nF capacitor.Quote from: hunter84 on October 02, 2014, 02:26:22 pmNo - have a read ofhttp://www.swarthmore.edu/NatSci/echeeve1/Ref/SingleSupply/SingleSupply.htmlYou need to invent a signal ground, at VCC/2 , and use that for your filters and amplifiers, when you're using single supply rails for opamps. What you're doing at the moment is much like using the -15V rail as ground in a classic split-rail circuit. You wouldn't do that!There are a bunch of links at the bottom of that page, that are probably worth a read. Single rail opamps require a fair amount of thinking about.Quote from: hunter84 on October 02, 2014, 02:26:22 pmAh - in that case, yes, you're doing about as well as I'd expect. Cool!

author: hunter84
author_rank: Newbie
timestamp: October 02, 2014, 05:50:39 pm
content: Thanks for your contribution. I have got the point you mean with virtual ground. Namely, it is putting an offset to the input signal. Moreover, you also say that use this virtual gnd for filtering purpose. OK, I will apply these. I hope that it will enhance the performance of my circuit.Also I will use AC coupling between two stages.I will share the results at the forum.