AD7993 datasheet(14/27 Pages) AD

14 / 27 page

AD7994/AD7993

–14–

REV. PrF

PRELIMINARYTECHNICAL DATA

Analog Input

Figure 6 shows an equivalent circuit of the analog input

sturcture of the AD7994/AD7993. 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

exceeds 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 Figure 6 is typically about 4pF and

can primarily be attributed to pin capacitance. The resis-

tor R1 is a lumped component made up of the on resis-

tance (RON) of a switch(track and hold switch) and also

includes the RON of the input multiplexer. The total resis-

tance is typically about 400

Ω. The capacitor C2 is the

ADC sampling capacitor and has a capacitance of 30 pF

typically.

For ac applications, removing high frequency components

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 sig-

nificantly 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 particular applica-

tion.

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 AD7994/AD7993 Typical Connection Diagram

VIN

VDD

16PF

4PF

CONVERSION PHASE - SWITCH OPEN

TRACK PHASE - SWITCH CLOSED

VDD

VIN1

GND

+5V SUPPLY

REF 19X

0.1µF

10µF

AD7994/

AD7993

0V to REFIN

INPUT

SDA

µC/µP

SCL

TWO WIRE SERIAL

INTERFACE

CONVST

ALERT

REFIN

SET TO REQUIRED

ADDRESS

VIN2

VIN3

VIN4

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