Solution
Understanding the ±10 V Specification on NI Modules
The ±10 V specification you see on modules refers to the voltage range relative to the module ground rather than earth ground. To fully understand what your module is capable of measuring, there are two important specifications to consider. We will use the NI 9215 module as an example.
The first specification to look at is Maximum Working Voltage. This is the voltage range within which your module can accurately read measurements. For the NI 9215, this voltage is listed as ±10.2 V of common. The common pin refers to the module’s common, not the earth ground.
This means that as long as the voltage you are trying to read on a channel is within ±10.2 V of the voltage level of the common pin, you will get accurate measurements in relation to the voltage on the common pin.
So, can we measure any 20V range with the 9215? Suppose you want to measure 400 V–420 V. This is where the second specification comes in: Channel-to-Earth Isolation. Channel-to-earth isolation defines how much voltage difference can exist between any channel and earth ground before the module can be damaged.
For the 9215, channel-to-earth isolation is rated at 250 Vrms when using screw terminals or 60 VDC when using BNC connectors. Therefore, 400 V would be out of range. However, you could comfortably measure a 200–220 VDC range using screw terminals.
Note that the module will return a voltage between ±10.2 V. You will need to add the supply voltage connected to the common pin to calculate the actual measured voltage.
Low Accuracy / High Voltage Workaround
Often, users tie the module’s common pin to earth ground, allowing measurement of any voltage within ±10.2 V of earth ground. However, tying the common pin to earth ground is not strictly required.
For example, you could set the common pin to 10 V using a 10 V power supply, connecting the positive lead of the supply to the common pin of the measurement module. In this case, the module could measure ±10.2 V with respect to 10 V—that is, -0.2 V to 20.2 V.
You must ensure that the 10 V supply connected to the common pin is accurate and stable, as all measurements will be relative to that voltage. If the supply drifts, your measurements will also drift.
High Accuracy / Low Voltage Workaround
Suppose the supply voltage connected to the common pin is noisy or slightly unstable. Each measurement sample would then be affected by the noise present in the supply at that moment.
If the supply voltage is 10 V or less, one strategy is to use a second measurement channel to monitor the supply voltage for every sample and incorporate it into the signal measurement.
To do this, connect the ground of the supply to the positive terminal of a second measurement channel, and short the negative terminal of that channel to the common pin.
On each sample, measure two channels: the signal channel and the supply ground channel. The supply ground channel will return a negative value, as its positive terminal is connected to the supply ground while its negative terminal is shorted to the common pin, which is tied to the supply voltage.
Using this method, you can measure a 14 V–18 V signal by subtracting the supply ground channel measurement from the signal channel measurement to obtain the actual voltage.
For example, if the supply ground channel measures -8.242 V and the signal channel measures 9.323 V, this indicates that the common pin was at 8.242 V for that sample, and the signal was 9.323 V higher than that. Therefore, the actual signal voltage is 9.323 - (-8.242) = 17.565 V at that sample.
As long as you continue to monitor the earth's ground on one of your channels, the power supply does not need to be perfectly accurate. It only needs to remain between 8–10 V for this example. The supply must be at least 8 V to measure up to 18 V accurately and less than 10 V to measure the supply ground correctly.