AD7810

–7–

REV. B

When using the pseudo differential input scheme, the signal on

conversion result will be incorrect. For single-ended operation,

pseudo differential input being used to make a unipolar dc cur-

rent measurement. A sense resistor is used to convert the current

to a voltage and the voltage, is applied to the differential input

as shown.

AD7810

Figure 9. DC Current Measurement Scheme

DC Acquisition Time

The ADC starts a new acquisition phase at the end of a conver-

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

end of a conversion there is a settling time associated with the

sampling circuit. This settling time lasts approximately 100 ns.

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 repre-

sents the source impedance of a buffer amplifier or resistive

network; R1 is an internal multiplexer resistance and C1 is the

sampling capacitor.

C1

3.5pF

R1

125

R2

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

lowing formula:

× (R2 + 125 Ω) × 3.5 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 approxi-

mately 4 ns. The charge time becomes significant for source

impedances of 2 k

Ω 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 will cause the THD

to degrade at high throughput rates. In addition, better perfor-

mance can generally be achieved by using an external 1 nF

ADC TRANSFER FUNCTION

The output coding of the AD7810 is straight binary. The

designed code transitions occur at successive integer LSB values

ideal transfer characteristic for the AD7810 is shown in Figure

11 below.

000...010

000...001

000...000

0V

ANALOG INPUT

111...111

111...110

111...000

011...111

1LSB

Figure 11. Transfer Characteristic