REV. A

AD7475/AD7495

–9–

ADC TRANSFER FUNCTION

The output coding of the AD7475/AD7495 is straight binary.

The designed code transitions occur midway between successive

integer LSB values (i.e., 1/2 LSB, 3/2 LSBs, etc.). The LSB size

AD7495 is shown in Figure 6 below.

111...111

111...110

111...000

011...111

000...010

000...001

000...000

0V 0.5LSB

ANALOG INPUT

Figure 6. AD7475/AD7495 Transfer Characteristic

TYPICAL CONNECTION DIAGRAM

Figure 7 and Figure 8 show a typical connection diagram for the

AD7475 and AD7495 respectively. In both setups the GND pin is

connected to the analog ground plane of the system. In Figure 7

REF IN is connected to a decoupled 2.5 V supply from a reference

source, the AD780, to provide an analog input range of 0 V to

pin of the AD7475 is connected to the same 3 V supply of the

microprocessor to allow a 3 V logic interface, see Digital Inputs

Section. In Figure 8, the REF OUT pin of the AD7495 is con-

nected to a buffer and then applied to a level-shifting circuit used

on the analog input to allow a bipolar signal to be applied to the

AD7495. A minimum 100 nF capacitance is required on the

REF OUT pin to GND. The conversion result from both ADCs is

output in a 16-bit word with four leading zeros followed by the

MSB of the 12-bit result. For applications where power con-

sumption is of concern, the power-down modes should be

used between conversions or bursts of several conversions to

improve power performance. See Modes of Operation section

of the data sheet.

GND

5V

SUPPLY

2.5V

AD780

3V

SUPPLY

AD7475

0V TO

2.5V

INPUT

SDATA

C/ P

SCLK

SERIAL

INTERFACE

0.1 F

(MIN)

REF IN

CS

0.1 F

10 F

0.1 F

10 F

Figure 7. AD7475 Typical Connection Diagram

Analog Input

Figure 9 shows an equivalent circuit of the analog input structure

of the AD7475/AD7495. 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

200 mV. This will cause these diodes to become forward-biased

and start conducting current into the substrate. 20 mA is the

maximum current these diodes can conduct without causing

irreversible damage to the part. The capacitor C1 in Figure 9 is

typically about 4 pF and can primarily be attributed to pin capaci-

tance. 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 capaci-

tance of 16 pF typically. For ac applications, removing high

frequency components from the analog input signal is recom-

mended by use of an RC low-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 signifi-

cantly 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 application.

R1

C2

16pF

D1

D2

C1

4pF

CONVERSION PHASE–SWITCH OPEN

TRACK PHASE–SWITCH CLOSED

Figure 9. Equivalent Analog Input Circuit

GND

5V

SUPPLY

3V

SUPPLY

AD7495

0V TO

2.5V

INPUT

SDATA

C/ P

SCLK

SERIAL

INTERFACE

0.1 F

(MIN)

REF OUT

CS

0.1 F

10 F

0.1 F

10 F

R

R

3R

R

V

0V

V

Figure 8. AD7495 Typical Connection Diagram