DC voltage on inputs of NI5761-2 ?

[Kees_D]

There is a dc voltage on the inputs of NI5761-2 (DC coupled) with no signal connected.

When I connect a voltmeter and a 50 ohm termination to the input of the NI5761-2,  I measure a voltage of 0.460V out of it.

Why is this, I expected something near zero volts.

[TomTr]

Hi Kees_D,

I'm wondering why you are using a 50 ohm termination with your voltmeter. Surely this means that you are measuring the voltage with a floating ground and therefore your reading is not actually the potential difference between your input and ground.

I might have misunderstood your set-up but it sounds to me that this could be the reason why you are reading this unexpected voltage is due to the floating ground.

Kind regards,

Tom

[Kees_D]

Hi TomTr,

Thanks for your answer. I use a SMA 50 ohm feed-throug termination  and connect the voltmeter between the inner and outer connection of the coax connector.

(The voltmeter is floating).  According to the specifications the input of the 5761 is single ended, so I suppose the outer conductor is grounded.

Maybe I am doing something wrong ?

[TomTr]

Hi Kees_D,

I understand your setup now. I think you are reading the voltage correctly.

This does sound odd. Is this affecting your readings at all?

Kind regards,

Tom

[Kees_D]

Hi TomT

The voltage is the same on all channels. Without the temination resistor I measure 1.24V.

This looks like a large input-biascurrent.

It does not affect the readings, but I like to have an explanation.

Thanks,

Kees

[Nathan-P]

Are you measuring 1.24V from the attached voltmeter, or from the 5761 device?  If this is the reading from the voltmeter, what is the reading from the 5761 device?

[Kees_D]

From the NI 5762 device I measure zero volts.

The voltage readings are on the voltmeter.

Regards,

Kees

[TomTr]

Hi Kees_D,

This is the expected behaviour for this device. It has a bias voltage of around 0.45V due to the ADC interface. The manual talks about this bias, in the note on page 17/

Kind regards,

Tom

[David-A · Accepted solution]

Hi Kees,

Certain FlexRIO Adapter Modules do not have front end conditioning that other instruments have that would pull the voltage as seen looking out of the analog input to zero. The 5761 does not have that conditioning so I would expect to see a non zero value if you hook up a voltmeter to the analog input like you've done. I guess the question here is why do you care what the floating voltage coming out of the analog input is? As long as the ADC values we are getting from the 5761 correctly reflect the values that the DUT is providing it, then its all good.

[Kees_D]

I understand that the input signal is connected directly to the ADC. After reading the datasheet of the ADC I understand it has something to do with the way the ADC is biased. 

OK, I can live with it. It does not affect the measurements.

Thanks for your answers.

Kees