Unaccepted Results from NI-9222 with BNC Connectors

[DukesHSG]

In our application, when no signal is applied to any of AI channels of NI-9222 (BNC) cards, they should default to ~ 0V but instead they are maxing out to either +10.7V or -10.7V. 

When I connect the signal cable, then they read the correct voltage reading which is ~ +2.5V in our application.

Important thing to note is that it is happening with newly arrived NI-9222 cards only and not with old batch of same card. Old cards default to ~0V when input signal is disconnected. 

NI tech support advised:

NI does not specify what voltage a device will read when there is no signal connected. If you want to see a 0V on the AI channel, there will need to be a zero-voltage signal applied across the + and - terminals of the channel, which is the same as grounding the channel. To pull down the signal to zero, you can use a pull-down resistor. Kindly check Can I Use Pull-Down Resistors for Analog Inputs? for more details.

My Question to community:

1. How do I use a pull-down resistor in BNC Connector type NI-9222 card? 
2. Why old NI-9222 cards read default no-signal value as zero but new one shows max values of either +10.7V or -10.7V ?

Thanks.

[rolfk]

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@DukesHSG wrote:

My Question to community:

1. How do I use a pull-down resistor in BNC Connector type NI-9222 card?

Well, do you know how to use a soldering iron or similar? You could make some cable or connector adapter with this built in

1. Why old NI-9222 cards read default no-signal value as zero but new one shows max values of either +10.7V or -10.7V ?

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Possibly they used a different but functionally compatible component in their product. The current chip situation often requires to evaluate new manufacturers, or look for alternative chips that function the same but might use a slightly different internal design in order to deal with product end of life announcements, delivery shortages and similar.

The input impedance of differential amplifiers used on those analog-digital converters are extremely high impedance and a slightly different internal transistor design can cause the according input bias current on an open input to drive the amplifier to one of the power rails. This is very normal and your previous situation where the module produced a 0V signal with open inputs was more of a bad coincidence (well good for you as you would like it to be like that, but bad in general as it made you believe that this is normal).

My guess is actually that they used some protection circuitry on each input channel that in the past did provide a low enough impedance path to ground to make the input look like being grounded when not connected and when replacing that protection circuitry with a new device, its impedance increased (which would be generally a good thing as a low impedance path causes a measurement fault) but that causes what you experience.