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2.6.3.6 - Signal Voltages Beyond 0-2.44 Volts (and Resistance Measurement)

The normal input range for a low voltage analog input channel (FIO/EIO) on the U3 is about 0-2.44 volts. There is also a Special 0-3.6V range available on those inputs. The easiest way to handle larger voltages is often by using the LJTick-Divider, which is a two channel buffered divider module that plugs into the U3 screw-terminals.

The basic way to handle higher unipolar voltages is with a resistive voltage divider. The following figure shows the resistive voltage divider assuming that the source voltage (Vin) is referred to the same ground as the U3 (GND).


Figure 2.6.3.6-1. Voltage Divider Circuit

The attenuation of this circuit is determined by the equation:

Vout = Vin * ( R2 / (R1+R2))

This divider is easily implemented by putting a resistor (R1) in series with the signal wire, and placing a second resistor (R2) from the AIN terminal to a GND terminal. To maintain specified analog input performance, R1 should not exceed the values specified in Appendix A, so R1 can generally be fixed at the max recommended value and R2 can be adjusted for the desired attenuation.

The divide by 2 configuration where R1 = R2 = 10 kΩ (max source impedance limit for low-voltage channels), presents a 20 kΩ load to the source, meaning that a 5 volt signal will have to be able to source/sink up to +250 µA. Some signal sources might require a load with higher resistance, in which case a buffer should be used. The following figure shows a resistive voltage divider followed by an op-amp configured as non-inverting unity-gain (i.e. a buffer).


Figure 2.6.3.6-2. Buffered Voltage Divider Circuit

The op-amp is chosen to have low input bias currents so that large resistors can be used in the voltage divider. For 0-5 volt applications, where the amp will be powered from Vs and GND, a good choice would be the OPA344 from Texas Instruments (ti.com). The OPA344 has a very small bias current that changes little across the entire voltage range. Note that when powering the amp from Vs and GND, the input and output to the op-amp is limited to that range, so if Vs is 4.8 volts your signal range will be 0-4.8 volts.

To handle bipolar voltages, you also need offset or level-shifting. Refer to application note SLOA097 from ti.com for more information.

The information above also applies to resistance measurement. A common way to measure resistance is to build a voltage divider as shown in Figure 2.6.3.6-1, where one of the resistors is known and the other is the unknown. If Vin is known and Vout is measured, the voltage divider equation can be rearranged to solve for the unknown resistance.

4 comments

Let's say I want to measure up to 150V absolute maximum, with nominal around 100V.

If I used R1 = 604Kohm and R2 = 10Kohm and ran this attenuated signal through the OPA344 unity gain opamp, do you see any issues with this?  Can you power the OPA344 from the U3?

Thanks,

Rod

 Sounds good.  680k would be a common resistor size you might use.  Yes, you can power the OPA344 from VS and GND from the U3.

You might want to consider an inexpensive isolator between the U3 and your host.  In the event you make a mistake and cause an overvoltage, you might damage the U3 but the isolator should protect your host.  The isolator will also break ground loops caused by the computer mains ground.  See the end of our USB via Other app note.

 

I have a sensor which gives a 5 V signal when a beam is blocked, and 0 mV if the beam is unbroken. The output are two wires (positive and ground), which together gives this 5/0 V behaviour.

I have a U3-HV. Originally I connected it to the high voltage input channels. However in that case I see a lot of high frequency noise. Normally I can remove this by determining the differential between both wires, but this is not allowed for the HV AIN.

Can I use a resistor circuit to reduce the voltage by a factor of 2.5 by a resistance circuit to allow differential measurements in the other FIOs? How should I connect the differential negative channel to the U3, as I don't have a GRND in this case (starting from figure 1)?

Thanks in advance

I think you are barking up the wrong tree.  A 5/0 signal should not have too much noise to discern whether it is high or low.  I suggest you start a forum topic so we can troubleshoot your noise problem.