Triggering off a 5 mV signal to deliver TTL-level outputs

Thread Starter

Strike-the-root

Joined Apr 10, 2019
14
We're using some fast (150 kHz) piezo pressure sensors mounted in line along a tube to measure the velocity of a pressure wave. The amplifier for these sensors has fixed gain and the output only jumps from 0 to 5 millivolts when the pressure wave passes. The timer-counters we're using require TTL-level inputs. I need to build an analog circuit that can trigger off the 5mV signal and deliver a high TTL output, somewhere between 2.7 and 5V. What would be a decent approach here?
 

Thread Starter

Strike-the-root

Joined Apr 10, 2019
14
Yes, of course we have components and an electronics bench. If the voltage outputs were a bit higher, this would be easy, but unfortunately, I've not been able to get an op amp to consistently trigger off the 5 mV signal.
 

crutschow

Joined Mar 14, 2008
25,986
For such a low signal you would likely need to amplify it with a low-offset, fast op amp, and then use a comparator on the output to generate the TTL signal.
What does the signal look like, i.e. shape, rise/fall times?
 
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Ian0

Joined Aug 7, 2020
1,113
Are you connecting to the piezo sensors themselves, or to some other preamp? I would have expected more than 5mV from the piezo itself. If there's only 5mV from the amplifier, it sounds like a poor amplifier to me.
Could we have a look at the spec for the piezo devices and the amplifier?
 

Thread Starter

Strike-the-root

Joined Apr 10, 2019
14
For such a low signal you would likely need to amplify it with a fast op amp, and then use a comparator on the output to generate the TTL signal.
What does the signal look like, i.e. shape, rise/fall times?
Here's a graph showing the output from one of the transducers. The output rises from the low level (~0V) to a higher level (8mV) in about 0.1 millisecond.

Edit: The lab has a high EMF background. 90% of the noise in the measurement is due to the lab environment, not the amp itself.
 

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Thread Starter

Strike-the-root

Joined Apr 10, 2019
14
Are you connecting to the piezo sensors themselves, or to some other preamp? I would have expected more than 5mV from the piezo itself. If there's only 5mV from the amplifier, it sounds like a poor amplifier to me.
Could we have a look at the spec for the piezo devices and the amplifier?
Piezo transducer connects to signal conditioner/amp. No other amps in the system.
See here: https://www.pcb.com/products?m=482c05
 

Thread Starter

Strike-the-root

Joined Apr 10, 2019
14
Presumably, then after the pressure wave hits, the signal returns to 0V - approximately how long does that take?
The piezo signal output will remain at the high level typically for at least 20 milliseconds. We have 4 of these sensors at positions A, B, C, D--so we can measure the time for the pressure wave to travel A-B, B-C, and C-D. Getting each output to trigger a high TTL-level is the main thing here.
 

Ian0

Joined Aug 7, 2020
1,113
I'm a little concerned about what might be in the "preamp"/"signal conditioner". It seems to require a load impedance of 1Meg.
The LT1212 with its 60nA of bias current might have an offset problem, as that would generate 60mV in a 1M source impedance.
Until I know what the "preamp"/"signal conditioner" really does, then I'd favour a jfet op-amp, with input bias currents down in the picoamp range, just in case.
 

Ian0

Joined Aug 7, 2020
1,113
Where did you see that?
On
https://www.pcb.com/products?m=482c05
footnote 1 says
"[1] With >= 1M ohm input impedance of readout device."

Not particularly clear what it means, but enough to sew the seeds of doubt in my mind.
It sort-of-implies that it should be driving a >1Meg input impedance, but maybe not.
If I had to design an interface without full knowledge of the "preamp". I'd choose a low-bias current op-amp.
I'm not even sure it is a preamp. What preamp would only have 5mV output?
On the other hand - it seems to have a 4mA output current to the sensor. Piezos don't need a bias current so what's that for? I've seen electrostatic transducers (Polaroid used to make them) which do need a bias current. Am I reading too much into this and seeing problems that don't exist?
 
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Ian0

Joined Aug 7, 2020
1,113
Any offset can be always be compensated in the comparator reference voltage
But it was a DC amplifier, with a gain of 20, with a maximum offset of125mV and an unspecified minimum which one would have to assume to be 0. It doesn't even say in the datasheet if the bias current varies with temperature, and I bet it does. The signal is only 5mV. It's going to need a preset, and it's going to be tricky to adjust.
 
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