PCIe-7842R Digital output voltage drops when driving external device

[iron_curtain]

Greetings,

We need to control a driver device (TTL high > +1.5V, Rin = 50ohm) at a rate of ~50kHz via one of the DIO ports of our R series DAQ PCIe-7842R. The DAQ is connected to the device through SHC68-68-RMIO cable and SCB-68A connector block. Prior to connecting the driver device to PCIe-7842R, the DIO maximum voltage was 3.2V. Once connected, the voltage dropped to 1.2V and could not trigger the driver device (the driver requires >1.5volts).

We also used the auxiliary power connector on the PCIe-7842R to increase the current limit of the board but it did not help. 

Can you please advise what the problem could be and how to solve it?

Thanks,

Bing

[TyVo]

Hi Bing,

Here is the manual for this family of products: http://www.ni.com/pdf/manuals/372492c.pdf

It is my understanding that the voltage level of the digital outputs is related to the amount and direction of the current. The ranges for these voltage levels are on page 4 of the manual. 

As a workaround to your current problem, perhaps you could include an amplifier circuit to boost the voltage where you need it?

-TyVo

[Henrik_Volkers · Accepted solution]

One DO of that card can drive 4mA according to the SPEC

If you need to drive a long cable (cable impedance) and/or load that current might not be sufficient.

Common solution: Add a TTL buffer/driver with a higher fan out at the breakout box.

Q&D tryout: According to the SPEC up to 8 DO are shortcut safe.. so if you use 2..4 DO  parallel  (from one port .. if that exists on an FPGA?) you might be able to drive your device. (They are not shortcutted if you set them to the same level at the same time, just in case 😉 )

[iron_curtain]

Hi TyVo and Henrik,

Thank you very much for the suggestions!

I am aware that the current output is limited to 4mA on the FPGA, that's why I attached a disk driver connector to the FPGA board to provide additional power (http://www.ni.com/datasheet/pdf/en/ds-98). Unfortunately it still didn't work (actually the measured output voltage didn't increase at all with the external power supply). 

For now, I agree with both of you that a TTL buffer could be the only option. I'll try that and report back.

Thanks again,

Bing

[Henrik_Volkers]

No matter how powerful your power supply is, the DO output current is limited by the design... tiny transistors inside that black bug 😉  output impedance ~100 ohm (?)