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Design >> Analog Design >> How to measure differential opamp's output?
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Message started by icfan on Apr 23^rd, 2005, 7:28pm

Title: How to measure differential opamp's output?
Post by icfan on Apr 23^rd, 2005, 7:28pm

I want to design a one stage differential opamp to sense some on-chip low voltage signals. The opamp's differential gain is to be 10dB around, the bandwidth is about 1GHz. Can anybody give suggestions how to send the opamp's differential outputs off the chip for measurement? What prober could be used for this measurement? How to simulate the prober's effect during the opamp design? Thank you in advance!

Title: Re: How to measure differential opamp's output?
Post by Paul on Apr 25^th, 2005, 12:31am

Hi,

In order to properly answer this question, you must give more information. In what set-up do you measure your chip: package, chip-on-board, wafer probing? Will you be able to use high-impedance probes (take care, their bandwidth is usually limited) or will you have 50-Ohm equipement (typical for RF measurements).

In the latter case, you cannot attach the measurement equipment directly to your amplifier output, except it has low-impedance output. Single-stage amplifiers usually are OTAs, not opamps and cannot drive such a low-impedance load. You need a buffer stage in between the OTA and the 50-Ohm equipment.

To simulate this, you simply have to include a model of the measurement equipment, typically a load resistor to ground with a load capacitor in parallel. You find the values for these components in your probe/scope data sheet.

Paul

Title: Re: How to measure differential opamp's output?
Post by icfan on Apr 25^th, 2005, 8:32am

Thank you Paul. I am going to use wafer probing for this measurement. 50-ohm probe head is to be used.
If the buffer is added, I guess, its output resistance should be set to 50-ohm for impedance matching. Am I right? Do I need to consider other issues?

Thanks a lot!

Title: Re: How to measure differential opamp's output?
Post by Paul on Apr 25^th, 2005, 11:57pm

If you work in a 50-Ohm environment, impedance matching should be guaranteed, but further more you have to simulate your circuit with the load. The resistive load may modify the operating point of your circuit, so be careful.

Regarding probing, you can use differential RF probes to directly sense the differential signal going off the chip, but this usually requires two scope channels. Converting it to single-ended eases the set-up but cancels the advantages of differential signalling.

Paul