AD7466/AD7467/AD7468

–10–

REV. PrC

Figure 11. REF193 as Power Supply to AD7466/AD7467/

AD7468

Analog Input

Figure 12 shows an equivalent circuit of the analog input

sturcture of the AD7466/AD7467/7468. 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 200mV. This

will cause these diodes to become forward biased and start

conducting current into the substrate. The capacitor C1 in

figure 12 is typically about 4pF and can primarily be at-

tributed to pin capacitance. The resistor R1 is a lumped

component made up of the on resistance of a switch. This

resistor is typically about 100

Ω. The capacitor C2 is the

ADC sampling capacitor and has a capacitance of 16pF

typically. For ac applications, removing high frequency

components from the analog input signal is recommended

by use of a 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

VIN

D1

VDD

D2

R1

C2

16PF

C1

4pF

CONVERSION PHASE - SWITCH OPEN

TRACK PHASE - SWITCH CLOSED

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 13 shows a graph of

the Total Harmonic Distortion vs. analog input signal

frequency for different source impedances when using a

supply voltage of 2.7V and sampling at a rate of 100

kSPS.

Figure 12. Equivalent Analog Input Circuit

VIN

GND

+5V

SUPPLY

0.1µF

10µF

REF193

0.1µF

1µF

TANT

+3V

AD7466/67/68

0V toVDD

INPUT

SDATA

µC/µP

SCLK

SERIAL

INTERFACE

CS

1mA

680nF

The AD7468 conversion result consists of four leading

zeros followed by the MSB of the 8-bit result.

Alternatively, because the supply current required by the

AD7466/AD7467/AD7468 is so low, a presision reference

can be used as the supply source to the AD7466/AD7467/

AD7468. A REF19x voltage reference (REF193 for 3V,

REF192 for 2.5 V ) can be used to supply the required

voltage to the ADC - see figure 11. This configuration is

especially useful if your power supply is quite noisy or if

the system supply voltages are at some value other than 3V

(e.g. 2.5V).

The REF19x will output a steady voltage to

the AD7466/AD7467/AD7468. In applications where

power consumption is important, the automatic power

down mode of the ADC and the sleep mode of the

REF19x reference should be used to improve power per-

formance. See Modes of Operation section of the

datasheet.

TYPICAL CONNECTION DIAGRAM

Figure 11 shows a typical connection diagram for the

AD7466 the conversion result is output in a 16-bit word

with four leading zeroes followed by the MSB of the 12-

bit result. The AD7467 conversion result consists of four

leading zeros followed by the MSB of the 10-bit result.

pecifications