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2.7.3.6 - Signal Voltages Beyond ±5 Volts (and Resistance Measurement)

The nominal maximum analog input voltage range for the UE9 is ±5 volts. 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 UE9 screw-terminals.

The basic way to handle higher voltages is with a resistive voltage divider. Figure 2.7.3.6-1 shows the resistive voltage divider assuming that the source voltage (Vin) is referred to the same ground as the UE9 (GND).


Figure 2.7.3.6-1. 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 10 kΩ, so R1 can generally be fixed at 10 kΩ and R2 can be adjusted for the desired attenuation. For instance, R1 = R2 = 10 kΩ provides a divide by 2, so a ±10 volt input will be scaled to ±5 volts and a 0-10 volt input will be scaled to 0-5 volts.

The divide by 2 configuration where R1 = R2 = 10 kΩ, presents a 20 kΩ load to the source, meaning that a ±10 volt signal will have to be able to source/sink up to ±500 µA. Some signal sources might require a load with higher resistance, in which case a buffer should be used. Figure 2.7.3.6-2 shows a resistive voltage divider followed by an op-amp configured as non-inverting unity-gain (i.e. a buffer).


Figure 2.7.3.6-2. 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. The LT1490A from Linear Technologies (linear.com) is a good choice for dual-supply applications. The LT1490A only draws 40 µA of supply current, thus many of these amps can be powered from the Vm+/Vm- supply on the UE9, and can pass signals in the ±5 volt range. Since the input bias current is only -1 nA, large divider resistors such as R1 = R2 = 470 kΩ will only cause an offset of about -470 µV, and yet present a load to the source of about 1 megaohm.

For 0-5 volt applications, where the amp will be powered from Vs and GND, the LT1490A is not the best choice. When the amplifier input voltage is within 800 mV of the positive supply, the bias current jumps from -1 nA to +25 nA, which with R1 = 470 kΩ will cause the offset to change from -470 µV to +12 mV. A better choice in this case 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. If this is a concern, use the external wall-wart to supply power to the UE9 as it typically keeps Vs around 5.2 volts.

The EB37 experiment board is handy for building these circuits.

The information above also applies to resistance measurement. A common way to measure resistance is to build a voltage divider as shown in Figure 2.7.3.6-1, 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.