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Output Amplifier Selection

Reference and Ground

Power Supply and Reference Bypassing

DAC8830-EP

DAC8831-EP

SGLS334C – AUGUST 2006 – REVISED APRIL 2007

For bipolar mode, a precision amplifier should be used, supplied from a dual power supply. This provides the

±V

REF output.

In a single-supply application, selection of a suitable operational amplifier may be more difficult because the

output swing of the amplifier does not usually include the negative rail; in this case, AGND. This output swing

can result in some degradation of the specified performance unless the application does not use codes near 0.

The selected operational amplifier needs to have low-offset voltage (the DAC LSB is 38

μV with a 2.5-V

reference), eliminating the need for output offset trims. Input bias current should also be low because the bias

current multiplied by the DAC output impedance (approximately 6.25 k

Ω) adds to the zero-code error.

Rail-to-rail input and output performance is required. For fast settling, the slew rate of the operational amplifier

should not impede the settling time of the DAC. Output impedance of the DAC is constant and

code-independent, but in order to minimize gain errors the input impedance of the output amplifier should be as

high as possible. The amplifier should also have a 3 dB bandwidth of 1 MHz or greater. The amplifier adds

another time constant to the system, thus increasing the settling time of the output. A higher 3-dB amplifier

bandwidth results in a shorter effective settling time of the combined DAC and amplifier.

Since the input impedance is code-dependent, the reference pin should be driven from a low impedance source.

The DAC8830 and DAC8831 operate with a voltage reference ranging from 1.25 V to VDD. References below

1.25 V result in reduced accuracy.

The DAC full-scale output voltage is determined by the reference. Table 1 and Table 2 outline the analog output

voltage for particular digital codes.

For optimum performance, Kelvin sense connections are provided on the DAC8831. If the application does not

require separate force and sense lines, they should be tied together close to the package to minimize voltage

drops between the package leads and the internal die.

For accurate high-resolution performance, it is recommended that the reference and supply pins be bypassed

with a 10

μF tantalum capacitor in parallel with a 0.1 μF ceramic capacitor.

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