Analog Input Pull Down Resistors

[wfotto]

For the USB-2416, and I assume most MCC DAQ devices, 100K pull down resistors are recommended to avoid floating inputs. This works when the input is from an active device, such as a conditioned instrumentation amplifier output. This does not work well with pure resistance devices such as mV output bridge circuits. Even though the 100K's are tied to the USB-2416 floating ground terminal, they are still tied across the bridge outputs. This adversely affects the output. As a workaround, we found 5M pull downs essentially remove the bridge interaction.

My question is, how do 5M pull downs affect the principle of floating inputs for the active device? Is there a maximum resistance, after which it doesn't work?

[JRys]

If you told me that you're using one input and it is connected to a load cell (bridge sensor), I would recommend connecting the minus excitation to the USB-2416 ground (GND) and not to use the pull down (reference) resistor. However, you may be also using grounded thermocouple and the combination can cause a ground loop. If you've found that a 5M ohm reference resistor works, then continue using it. It's large enough that I don't expect it to affect the reading. 

[wfotto]

JRys,

For the load cell (full bridge): our system can record up to 10 load cells at a time. The load cell is excited with 10VDC. Isn't the USB-2416 GND isolated, therefore no current return path to the power supply?

Yes, the 5M works for a load cell configuration. But is that OK with an active device, one with an op amp output? Will that high impedance cause issues, like bias currents in the USB-2416 instrumentation amp?

Thanks,

Bill

[JRys]

The USB-2416 is isolated from earth ground. When using an isolated sensor and a differential input, a reference resistor must be used to provide a reference for the USB-2416 A/D converter (GND). The resistor is connected from the low side to ground. Otherwise the measurement floats and you get inconsistent results. Usually, Load Cells don't need a reference resistor because the internal bridge circuit provides a path back to minus excitation, which tends to be ground. If you're using reference resistors because the measurements float, then I suspect the excitation power is isolated from earth ground (and isolates the load cells).  Do you use a single power supply for all 10 Load Cells, and does it have a (+), a (-), and a ground terminal?  The three terminals typically mean that (+) and(-) are isolated.

Lastly, a typical load cell does not have an op amp output unless you're also using load cell amplifiers.

 

 

[wfotto]

Maybe a drawing would help. We started with no pull down resistors. We were getting some erratic data with our High Level transducers, so changed to the 2416 recommendation of 100K pull downs. Unfortunately, this configuration, with our Low Level transducer, put the two 100Ks in parallel with the bridge 5K strain gages. The transducer output at full pressure was about 600uV lower than without the pull downs (nominal 0-30mV). I'm not sure if this was due to just the parallel combination, or if the DAQ DGND isn't totally isolated from the P GND? When we tested with the Low Levels with the 5M pull downs, they didn't affect the Low Level output at full pressure, basically the same as no pull downs.

The concern is whether the 5Ms will adversely affect the High Level transducers? Note, the High Level transducers, on the Amp Board side, is a load cell with an instrumentation amplifier.

DAQ Hi_Lo Level.pdf

[JRys]

A picture is worth a thousand words...

Let's look at the low level setup first. Because you can trace a path from the USB-2416 low side input, back through the bridge to your power supply, there's no need for reference resistors. The power supply ground is the system ground. Set the USB-2416 input to differential mode, set the Data Rate property to 60 Hz and use the 78mV range.  I want to emphasize the Data Rate setting. it will make the USB-2416 A/D converter oversample the input to produce a better measurement. Think of it as a low pass filter. If you find that the measurement drifts, connect the power supply minus (-) to GND.

Now let's look at the high level configuration. Remove any additional resistors. Configure the USB-2416 input for single-ended mode, data rate set to 60 Hz and the 5 volt input range. Connect the amplifier output signal to CH0H and the amplifier ground to GND. Single-ended inputs for channels 0-9 use the high side terminals CH0H - CH9H.

[wfotto]

Just to clarify, the Hi Level output is floating. There's a DC-DC board (28V to 12V converter) and transformer isolated from the Amb board. The "V" (virtual) ground is floating, not tied to any other ground except at the DAQ. Ergo, we need differential DAQ inputs.