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Measurements >> Other Measurements >> Measurement of output current of an OTA
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Message started by rajdeep on Jul 22^nd, 2011, 6:00am

Title: Measurement of output current of an OTA
Post by rajdeep on Jul 22^nd, 2011, 6:00am

Hi All,

How to measure output current of a single-ended OTA with push-pull kind output stage during test (in ATE)?

More specifically I would like to measure the output current of a single-ended OTA during test and then trim something according to that.
Now the current can be both positive (source) as well as negative (sink) depending on the polarity of the input differential voltage.
So I wonder whether the test equipment would be smart enough to sense this current without forcing any voltage source at that node.

I am waiting for the comments from our expert test enggs but they all seem to have gone for vacation in summer!! :-|

Regards,
Rajdeep

Title: Re: Measurement of output current of an OTA
Post by boe on Jul 22^nd, 2011, 7:13am

Hi Rajdeep,

rajdeep wrote on Jul 22^nd, 2011, 6:00am:

...
So I wonder whether the test equipment would be smart enough to sense this current without forcing any voltage source at that node. ...

If your output sources current, you need a sink for it in the ATE - so you need a load (voltage source).

B O E

Title: Re: Measurement of output current of an OTA
Post by rajdeep on Jul 22^nd, 2011, 7:31am

The problem is it could be a sink as well, i.e. the current can change its direction. So the ATE needs to figure that out and measure the current!

A simple solution could be to add a dc current to it to ensure it
always remains positive OR negative. But then this dc current needs to be trimmed for accuracy as well unless the measurement instrument does this shifting internally. I doubt that.

Regards,
Rajdeep

Title: Re: Measurement of output current of an OTA
Post by boe on Jul 22^nd, 2011, 10:13am

Rajdeep,
The sign of the current is not the issue I was aiming at.

If your OTA drives a current (of either sign) the ATE pin will see the same current in opposite direction. So unless you have a path to GND, your current will charge the pin cap to saturation and then go off.

B O E