# Wrong voltage measured with a DAQ in presence of a RC filter

I'm using a DAQ from National Instruments. Testing my LabView software I noticed that when I plug a 1.5V DC battery in the analog input channels if I connect an RC filter (R=56 kΩ, C=10 uF) to the batter then my DAQ instead of measuring the expected voltage it measures a lower value, 1.22V, as if there was a load of about 300 kΩ. The declared imput impedance is instead 144 kΩ and this happens only if I measure the voltage in differential mode, if I measure it in RSE (Reference Single Ended) that doesn't happen (i.e. I read 1.5V). Can you explain why?

Furthermore: in the lower schematic, what is the purpose of the +2.5 Vref and of the resistors?

• Have you measured the input impedance of your DAQ? Perhaps it's 300 kΩ. – Harry Svensson Nov 17 '17 at 12:08
• The declared imput impedance is 144 kΩ, but if I look to the analog input circuitry (the pic above) I see something different. Furthermore why should this phenomenon be present only in differential mode? – Alessandro Zunino Nov 17 '17 at 12:10

$$R_{in} = 127 k\Omega + (30.9 k\Omega || 39.2 k\Omega) = 127k\Omega + \frac{(30.9)(39.2)}{30.9 + 39.2} k\Omega \approx 144.28k\Omega$$