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2.7.1 - Typical Analog Output Connections [U6 Datasheet]
2.7.1.1 - High Current Output [U6 Datasheet]
The DACs on the U6 can output quite a bit of current, but have 50 Ω of source impedance that will cause voltage drop. To avoid this voltage drop, an op-amp can be used to buffer the output, such as the non-inverting configuration shown in Figure 2.6-1. A simple RC filter can be added between the DAC output and the amp input for further noise reduction. Note that the ability of the amp to source/sink current near the power rails must still be considered. A possible op-amp choice would be the TLV246x family (ti.com).
2.7.1.2 - Different Output Ranges [U6 Datasheet]
Consider using an LJTick-DAC rather than the built-in DACs. The LJTDAC provides +/-10 volt outputs with 14-bit resolution across that span.
The typical output range of the DACs is about 0.04 to 4.95 volts (assuming Vs=5.0V). For other unipolar output ranges, an op-amp in the non-inverting configuration (Figure 2.6-1 in Section 2.6.3.5) can be used to provide the desired gain. Connect Vin in Figure 2-3 to the DACx terminal, connect -V to GND, connect +V to a voltage source greater than the max desired output, and set R1 & R2 to control the gain. Example: Unipolar 0-10 volt output. Use R1=93.1kΩ and R2=100kΩ to provide a gain of about x1.07, and use Vm+ to provide up to 2.5mA of 12V power to the LT1490A op-amp. This will provide a nominal output range of 0.08 to 10.3 volts.
For bipolar output ranges, such as ±10 volts, a similar op-amp circuit can be used to provide gain and offset, and of course the op-amp must be powered with supplies greater than the desired output range (depending on the ability of the op-amp to drive it's outputs close to the power rails). For example, the Vm+/Vm- supplies available from the U6 are typically ±13 volts. If these supplies are used to power the LT1490A op-amp (linear.com), which has rail-to-rail capabilities, the outputs could be driven very close to ±13 volts.
For bipolar ranges a reference voltage is also required to provide the offset/level-shifting. In the following circuit, DAC1 is used to provide a reference voltage. The actual value of DAC1 can be adjusted such that the circuit output is 0 volts at the DAC0 mid-scale voltage, and the value of R1 can be adjusted to get the desired gain. A fixed reference (such as 2.5 volts) could also be used instead of DAC1.
Figure 2.7.1.2-1. ±10 Volt DAC Output Circuit
A two-point calibration should be done to determine the exact input/output relationship of this circuit. Refer to application note SLOA097 from ti.com for further information about gain and offset design with op-amps.