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Design >> RF Design >> OPAMP P1DB IIP3 measurement & simulation ??
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Message started by fassage on Oct 31^st, 2011, 4:11am

Title: OPAMP P1DB IIP3 measurement & simulation ??
Post by fassage on Oct 31^st, 2011, 4:11am

Hi All,

Im designing a 7th order active RC filter, and need to measure its linearity, in order to compare with current published figures. I have the following 3 questions please.

1. Does it make any sense to simulate the OPAMP linearity in open loop ?

2. Does the IIP3 or P1DB of the OPAMP influence the respective performances of the overall filter ?

3. Almost all literature refer to compression point and intercept point measurements in terms of input&output power (dBm).
(a) When simulating a simple 2stage opamp, that has been designed to drive a capacitive load, how do you setup the simulation for input & output power ?
(b) otherwise how would it be done for the overall filter ?

Many thanks for your valuable time and support for this question in advance.

Thanks

Title: Re: OPAMP P1DB IIP3 measurement & simulation ??
Post by loose-electron on Oct 31^st, 2011, 3:32pm

the bandwidth and open loop gain of the op-amp will be the primary controls of closed loop linearity.

Open loop linearity of an op-amp is not too meaningful.

Title: Re: OPAMP P1DB IIP3 measurement & simulation ??
Post by rfcooltools.com on Oct 31^st, 2011, 4:35pm

fassage,

As loose-electron stated the opamps open loop response nothing like the closed loop response so it does not make sense to do so. When the opamp loop is closed the input will look like an AC ground at least until the gain and bandwidth deteriorates it to impedance greater than that.

Unless your circuit drives a real impedance then the output power is not what you are looking for. In your circuit where the load is a capacitor so you will want to observe the distortion in voltage at the output referred back to the input ( provided that the circuit is being driven by a matched source). If this block is in the interior of a system on chip then IIP3 and P1dB are not the real specs but rather an "equivalent" voltage at the input "as if" it was a matched system of some impedance.

If you are trying to save current and not designing the opAmp to meet the loop bandwidth, gain and drive strength required to achieve distortion levels from feedback then:
To simulate the distortion of a single stage of your filter you will want to drive it with current source and load it with the next stage as an out put.
Then you will need to map a transfer function of current at its input to the previous stage current when it drives it when the entire filter is connected and driven with a power or power equivalent voltage.
Finally measure the the output voltage with distortion and map it back to the filter input using the previous transfer function combined with the opamp being tested current to voltage transfer function.

You can obtain the transfer function initially by using ideal opamps with the filter R's and C's.

http://rfcooltools.com