Slow AC voltage measurement readings with hp34401a

[TanWal]

Hi,

I am currently using the hp34401a to record an input and output signal voltage (this is then converted programmatically to frequency response). This measurement however I consider to take exceptionally long for what it is (approx 3 seconds per measurement). I am using Configure measurement.vi (set to AC voltage on auto range) and Read(single point).vi. If anyone can give me some ideas how to speed up these readings that would be appreciated.

I have had a look at a similar test done with Labview 6.2.1, where the testing is much faster. The same instrument is being used but with Traditional NI-DAQ.

Thanks

Tania

Note: Labview 8.6

[Dennis_Knutson]

First, don't use autoscale if you want a fast measurement. Second, lower the resolution. Third, check the aperture time. Fourth, lower the integration time.

[TanWal]

I thought that was optimisation for DC/ resistance measurements?

[Dennis_Knutson]

I should not have mentioned aperature since that is for frequency and period only. All others apply.

[Henrik_Volkers]

double

[Henrik_Volkers]

Take a look into the specs.

As far as I remember the AC measurements take much longer due to the rms conversion that has some filters.I assume you switch between input and output of your DUT, so after every switch the rms filter needs to settle.

I don't know your frequency range and the max possible direct or subsample capability of the 34401 (I use the 3458 🙂 ) but maybe you are faster in the sampling mode , capturing 1 to 10 periods and do a sinus approximation. Big advantage: You also get the phase when triggered by the signal source.

[TanWal]

I have tried all the possible/ suggested ways I can think of to improve the reading. There seems to be little that can be done in AC. Even applying the fast filter did nothing to improve the time. I tried to do DC voltage measurement, then take a multipt read and use the Basic rms.vi to obtain an AC value but this is very inaccurate. It seems that this DMM is very limited in its speed for this type of measurment.

[Dennis_Knutson]

I'll try to setup something next time I have access to the equipment. I'm not doing single point. I am doing multipoint and I get a reading about every 3ms.

Also, post your code. I would hope that the only thing in your loop is the read.

[Henrik_Volkers]

---

@TanWal wrote:

I have tried all the possible/ suggested ways I can think of to improve the reading. There seems to be little that can be done in AC. Even applying the fast filter did nothing to improve the time. I tried to do DC voltage measurement, then take a multipt read and use the Basic rms.vi to obtain an AC value but this is very inaccurate. It seems that this DMM is very limited in its speed for this type of measurment.

---

I took a look in the spec. Forget the DC way if you have frequencies higher 80Hz....however if you have sampled waveforms do a tone detection (FFT based) or better a SAM (sinus approximation via min MSE) to get the best out of your data ( For FRF with sinus excitation). With RMS measurements you will measure hum and noise too, however you didn't tell us the uncertainty goal nor your DUT.

I don't use this instrument but from the spec and my experience with HPs(agilent) you should be able to get 10 reading/s in AC mode BUT still have to wait for the settling of the AC filter after switching (I assume you use the front and rear input)

I would encourage you to write a small state machine for the task and post it here.  

[TanWal]

This is the general flow:

1. Generate sinusoidal input signal to DUT using hp33120a

2. Connect Signal Generator (hp33120A) to DMM by switching relays

3. Measure input signal (AC Voltage single point) (hp34401a)

4. Switch relays for DMM read output signal of DUT

5. Measure output signal (AC Voltage single point) (hp34401a)

6. Perform arithmetic to convert to dB (20xlogVo/Vin)