LabJack UE9 User Manual page 12

There are numerous criteria used to choose an op-amp from the thousands that are available. One of the main criteria is that the
op-amp can handle the input and output signal range. Often, a single-supply rail-to-rail input and output (RIRO) is used as it can be
powered from Vs and GND and pass signals within the range 0-Vs. The OPA344 from Texas Instruments (ti.com) is good for many
5 volt applications. The max supply rating for the OPA344 is 5.5 volts, so for applications using Vm+/Vm- (~12 volts) or using the
±10 volt supply on the EB37, the LT1490A from Linear Technologies (linear.com) might be a good option.
The op-amp is used to amplify (and buffer) a signal that is referred to the same ground as the LabJack (single-ended). If instead
the signal is differential (i.e. there is a positive and negative signal both of which are different than ground), an instrumentation
amplifier (in-amp) should be used. An in-amp converts a differential signal to single-ended, and generally has a simple method to
set gain.
The EB37 experiment board is handy for building these circuits.
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. The following figure shows the resistive voltage divider
assuming that the source voltage (Vin) is referred to the same ground as the UE9 (GND).
Figure 2-4. 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. The following figure shows a resistive voltage divider followed by an op-amp configured as non-inverting unity-
gain (i.e. a buffer).
Figure 2-5. 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.
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