Data Sheet

AD8628/AD8629/AD8630

FUNCTIONAL DESCRIPTION

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and PSRR help to maintain the overall circuit accuracy. As long

as the rate of change of the current is not too fast, an auto-zero

amplifier can be used with excellent results.

OUTPUT AMPLIFIER FOR HIGH PRECISION

DACS

The AD8628/AD8629/AD8630 are used as output amplifiers for a

16-bit high precision DAC in a unipolar configuration. In this case,

the selected op amp needs to have a very low offset voltage (the

DAC LSB is 38 µV when operated with a 2.5 V reference) to

eliminate the need for output offset trims. The input bias current

(typically a few tens of picoamperes) must also be very low be-

cause it generates an additional zero code error when multiplied by

the DAC output impedance (approximately 6 kΩ).

Rail-to-rail input and output provide full-scale output with very little

error. The output impedance of the DAC is constant and code

independent, but the high input impedance of the AD8628/AD8629/

AD8630 minimizes gain errors. The wide bandwidth of the amplifi-

ers also serves well in this case. The amplifiers, with settling time

of 1 µs, add another time constant to the system, increasing the

settling time of the output. The settling time of the AD5541 is 1 µs.

The combined settling time is approximately 1.4 µs, as can be

derived from the following equation:

tS TOTAL = tSDAC2+ tSAD86282 (1)

Figure 64. AD8628 Used as an Output Amplifier