QA403 1dbV clipping


I’m probably doing something wrong but by having a simple loopback, it seems that the maximum level the 403 can handle is 0dbV. What I mean is that if I set the GEN to 0dbV and the attenuation to 0dBV everything looks OK.

However, once I increase the GEN1 to +1dBV, the input gets overdriven/ distorted.

Anything above that (i.e +2dBV upwards) and the relays are constantly triggered.

Please see the attached images.

This is brand new unit. What am I doing wrong here?

0 dBV

+1 dBV

Hello peppermint
This behavior is normal. If your input is set to 0dBV, a 0dBV (1Vrms) signal drives the ADC to full scale. If the signal level goes beyond 0dBV, the ADC is in overload.
The input level setting should always be higher than the actual signal level.

To add to the previous post, I followed the calibration and the levels seem OK on my DVM (both
at 60Hz and at 1kHz) and no DC apparent.

Also, here are the two results. First image is when the unit was turned on and the second image is after a few hours of running. Do they look OK?

On start up

after a few hours of running


Thank you for clarifying.

Excuse my ignorance but I’m not sure what you mean by “The input level setting should always be higher than the actual signal level.”

Hello peppermint
What I mean is, that your input signal should be below the setting. If your input is set to 0dBV, signals of -5dBV or -1dBV are measured without distortion. If you want to measure a +10dBV signal, your input setting should be +12dBV or +18dBV.
I hope this clarifies.

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You can always think of the ranges in Volts if that makes it easier. Your DMM probably auto ranges, so you might not think about “ranges”, but each range on the QA403 is the maximum voltage you can apply before overloading the ADC. The ranges are there to ensure you can apply a wide range of voltages to the device and still keep the ADC in an optimum. The QA403 doesn’t autorange, you need to set the maximum before applying a signal.

You’ll notice the QA403 always defaults on startup to the highest (most attenuated) range. That is the safest, but also the noisiest range.

If you want to convert the dBV ranges to volts, pull out your scientific calculator or use this:

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