MCC 128 DAQ Issue (unwanted voltage increase on other channel)

[angelo_b]

Hello,
MCC_128_DAQ_Issue.pdf
We have a few of your MCC 128 DAQ and we encountered an issue that we don’t know how to resolve. See attached pdf for more detail. When 3V (or any voltage) is applied to CH0 of the DAQ, the adjacent channel (CH1) also increases to 0.38 V. Similarly, when the 3V is applied to CH1, CH2 also increases to 0.38V.

We then added a 0.1uF as shown in the PDF. The noise is reduced, but the higher the sampling rate, the higher the increase on the next channel also occurs.

Do you know what the issue is? Is there anything wrong with our setup?

Any help would be appreciated!

 

Thank you!

[DAQman]

The MCC 128 uses analog multiplexing to gate each channel, one by one, to the A/D converter. Internally, the input multiplexer (DG508) has a capacitance between each channel. When it switches, the voltage difference from one channel to the next appears as an AC signal and passes through to the next channel. However, as long as the input signal has a low impedance, the charge dissipates, allowing the input signal to charge the input to the correct value before the following A/D conversion. Long story short, do not scan open channels or use resistor dividers. The recommended signal source impedance is 100 ohm or less. 

[angelo_b]

Thank you, JRys! We are trying to measure DC voltages 0V to 100V, so we have an attenuator by using 900K and 100K ohm to attenuate the DC signal coming in when we saw this bleed. 

Could this be fixed if we add an op amp buffer in between the source (with attenuator) and the MCC 128?  Do you have other recommendations? 

Thank you again!

[DAQman]

An op amp is an excellent way to convert high impedance to low.

Another option is to use twice as many channels. Connect your signals to channels 0 & 2, connect channels 1 & 3 to ground, and scan all four simultaneously. The channels shorted to ground will dissipate the cross-talk caused by your resistor dividers.