Author Topic: Odd DC-offset in comparator circuit with ultrasonic receiver  (Read 1967 times)

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Offline MBVTopic starter

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Odd DC-offset in comparator circuit with ultrasonic receiver
« on: November 01, 2018, 12:03:40 am »
Hi,

I have the following ciruit:


The ultrasonic receiver is putting out a 40kHz sinewave with about 200-1000mVpp depending on the distance from the transmitter. My goal is to use the comparator to convert the signal into a square wave which can be fed into a microcontroller for phase difference measurements between channels. A zero crossing detector basically. I was expecting to get a 50% duty cycle square wave at the comparator output, but instead I'm getting like a 70% duty cycle sq wave. When troubleshooting I found out that the sine wave from the receiver, when measured directly at the receiver pins has a significant DC offset (varies with signal amplitude, but its roughly 100-200mV), which of course explains the output of the comparator. Where in the world does this DC component come from though? When I disconnect the receiver from the circuit and measure it on its own, it of course has a nice centered sinewave on its output...

Components used:

- both resistors 1kohm
- comparator MAX944

Any help with this is appreciated!
 

Offline Audioguru

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Re: Odd DC-offset in comparator circuit with ultrasonic receiver
« Reply #1 on: November 01, 2018, 12:12:56 am »
1) The (+) input of your comparator does not have any DC bias voltage if the receiver does not give it any (if the receiver is a piezo disc).
2) The input offset voltage of the comparator is amplified a few hundred thousand times by the comparator.

If you use an opamp with a reasonable gain of maybe 100 and bias the opamp correctly then you can capacitor-couple the receiver to the opamp.
 

Offline MBVTopic starter

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Re: Odd DC-offset in comparator circuit with ultrasonic receiver
« Reply #2 on: November 01, 2018, 12:28:03 am »
Thanks! Actually didn't realize I was using a piezo tranceiver.

Could I capacitor couple the receiver to the comparator though, if I bias the positive input to 3.3/2 V using some significantly higher value resistors (like 100k)?
 


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