AD7817ARZ datasheet(13/20 Pages) AD

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Data Sheet

AD7817/AD7818

Rev. D | Page 13 of 20

DC Acquisition Time

The ADC starts a new acquisition phase at the end of a conversion

and ends on the falling edge of the CONVST signal. At the end

of a conversion, a settling time is associated with the sampling

circuit. This settling time lasts approximately 100 ns. The

analog signal on VIN is also being acquired during this settling

time. Therefore, the minimum acquisition time needed is

approximately 100 ns.

Figure 10 shows the equivalent charging circuit for the sampling

capacitor when the ADC is in its acquisition phase. R2 represents

the source impedance of a buffer amplifier or resistive network,

R1 is an internal multiplexer resistance, and C1 is the sampling

capacitor.

VIN

3pF

1kΩ

Figure 10. Equivalent Sampling Circuit

During the acquisition phase, the sampling capacitor must be

charged to within a 1/2 LSB of its final value. The time it takes

to charge the sampling capacitor (TCHARGE) is given by

TCHARGE = 7.6 × (R2 + 1 kΩ) × 3 pF

For small values of source impedance, the settling time associated

with the sampling circuit (100 ns) is, in effect, the acquisition

time of the ADC. For example, with a source impedance (R2) of

10 Ω, the charge time for the sampling capacitor is approximately

23 ns. The charge time becomes significant for source impedances

of 1 kΩ and greater.

AC Acquisition Time

In ac applications, it is recommended to always buffer analog

input signals. The source impedance of the drive circuitry must

be kept as low as possible to minimize the acquisition time of

the ADC. Large values of source impedance cause the THD to

degrade at high throughput rates.

ON-CHIP REFERENCE

The AD7817/AD7818 have an on-chip, 1.2 V band gap reference

that is gained up to give an output of 2.5 V. By connecting the

REFIN pin to analog ground, the on-chip reference is selected.

This selection causes SW1 to open and the reference amplifier

to power up during a conversion (see Figure 11). Therefore, the

on-chip reference is not available externally. An external 2.5 V

reference can be connected to the REFIN pin, which has the

effect of shutting down the on-chip reference circuitry and

reducing IDD by approximately 0.25 mA.

1.2V

REFIN

SW1

2.5V

EXTERNAL

REFERENCE

DETECT

BUFFER

1.2V

26kΩ

24kΩ

Figure 11. On-Chip Reference

ADC TRANSFER FUNCTION

The output coding of the AD7817/AD7818 is straight binary. The

designed code transitions occur at successive integer LSB values

(that is, 1 LSB, 2 LSBs, and so on). The LSB size is = 2.5 V/1024 =

2.44 mV. The ideal transfer characteristic is shown in Figure 12.

ANALOG INPUT

1LSB

+2.5V × 1LSB

1LSB = 2.5/1024

2.44mV

111...111

111...110

111...000

011...111

000...010

000...001

000...000

Figure 12. ADC Transfer Function

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