Use 2.5V Output as reference for AO on USB6008

[bobikobi]

Hi,

I want to control a scanner with USB 6008 AOs.

The scanner is working with differential signal. To get a range of -2.5V to 2.5V i want to use a reference of 2.5V against the AO signal.

Is it possible to use the USB6008 2.5V souce to do so? Or will I need external reference?

Bobi

[bobikobi]

To make the question more simple:

Does anyone have experience using the 2.5V output in an application of any sort?

Can it harm the usb6008 to use it? In the manual it is listed for self testing of usb6008.

[agnes88 · Accepted solution]

Hi,

I'm not sure whether I got your point. Could you send a schematic of the circuit you want to make?

However, I'll try to help you. From what I understood, you want to provide the scanner's input with the voltage +/- 2.5 V, but AO of the USB6008 has only the range of 0-5 V. Your idea is to wire one of the scanner's inputs to the external voltage reference of +2.5 V (pin 30 of USB6008) and the other input to the AO channel (0-5V) of USB6008. 

If I got your idea right, it is possible, but strongly not recommended. As you mentioned "+2.5 V External Reference" is used for self-test, which means it gives us high-purity reference voltage (which is very useful while checking if hardware is not malfunctioned), but it may not accept currents exceeding a certain value (such currents could demage the device). In your case, the current flowing through the circuit depends on the scanner's impedance and voltage applied to it, so we aren't able to establish whether it'll be harmful to the device or not. Moreover, I could not find any information about the maximum current value acceptable by "+2.5 V External Reference" output.

A possible solution to your case may be using two Analog Outputs from USB 6008, e.g.:

- AO0: set as +2.5 VDC reference,
- AO1: set as a voltage changing in range 0-5 V.

Ground of the scanner should be connected to the GND of USB6008.

If you have some questions, let me know.

[Henrik_Volkers]

The specs are here

1mA ...

[bobikobi]

Thank you for your input.

@agnes88 You got the idea right. Only I can not use one of the AO as reference since I will need both for the scanner (x and y position).

In the meantime I connected the 2.5 V output as reference and it is working good. The current was measured with a few µA. It is a good solution so far and I don't see a reason for not using it for now.

Connecting GND of Scanner to USB6008 actually crashes the NI Device so I am using GND of Scanner Power Source (No clue why that happens since it is the same GND actually).

Greetings