AD7992 datasheet(11/21 Pages) AD

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AD7992

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REV. PrH

PRELIMINARYTECHNICAL DATA

Analog Input

Figure 6 shows an equivalent circuit of the analog input

sturcture of the AD7992. The two diodes D1 and D2

provide ESD protection for the analog inputs. Care must

be taken to ensure that the analog input signal never ex-

ceeds the supply rails by more than 300mV. This will

cause these diodes to become forward biased and start

conducting current into the substrate. 10 mA is the maxi-

mum current these diodes can conduct without causing

irreversable damage to the part. The capacitor C1 in Fig-

ure 6 is typically about 4pF and can primarily be attrib-

uted to pin capacitance. The resistor R1 is a lumped

component made up of the on resistance (RON) of a switch

(track and hold switch) and also includes the RON of the

input multiplexer. This resistor is typically about 100

Ω.

The capacitor C2 is the ADC sampling capacitor and has

a capacitance of 30 pF typically.

For ac applications, removing high frequency compo-

nents from the analog input signal is recommended by use

of an RC band-pass filter on the relevant analog input pin.

In applications where harmonic distortion and signal to

noise ratio are critical the analog input should be driven

from a low impedance source. Large source impedances

will significantly affect the ac performance of the ADC.

This may necessitate the use of an input buffer amplifier.

The choice of the op amp will be a function of the par-

ticular application.

When no amplifier is used to drive the analog input the

source impedance should be limited to low values. The

maximum source impedance will depend on the amount of

total harmonic distortion (THD) that can be tolerated.

The THD will increase as the source impedance increases

and performance will degrade. Figure 7 shows a graph of

the Total Harmonic Distortion vs. analog input signal

frequency for different source impedances when using a

supply voltage of 3V±10% and 5V ±10% and sampling at

a rate of xkSPS. Figure 8 shows a graph of the total har-

monic distortion versus analog input signal frequency for

various supply voltages while sampling at xkSPS .

Figure 6. Equivalent Analog Input Circuit

Figure 7. THD vs. Analog Input Frequency for

Various Source Impedance for VDD= 3V and 5V

Figure 8. THD vs. Analog Input Frequency,

Fs = xkSPS

Figure 5 AD7992 Typical Connection Diagram

VDD

VINx

GND

+5V SUPPLY

REF 19X

0.1µF

10µF

AD7992

0V to REFIN

INPUT

SDA

µC/µP

SCL

TWO WIRE SERIAL

INTERFACE

CONVST

0.1µF

ALERT

REF IN

SET TO REQUIRED

ADDRESS

VIN

VDD

30PF

4PF

CONVERSION PHASE - SWITCH OPEN

TRACK PHASE - SWITCH CLOSED

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