Data Sheet

AD7452

Rev. C | Page 15 of 24

and minimum common-mode voltages for each reference value

according to Figure 23.

COMMON-MODE (CM)

2V p-p

REFERENCE = 2V

COMMON-MODE (CM)

2.5V p-p

REFERENCE = 2.5V

Analog Input Structure

Figure 26 shows the equivalent circuit of the analog input

structure of the AD7452. The four diodes provide ESD

protection for the analog inputs. Care must be taken to ensure

that the analog input signals never exceed the supply rails by

more than 300 mV. This causes these diodes to become

forward-biased and start conducting into the substrate. These

diodes can conduct up to 10 mA without causing irreversible

damage to the part. The capacitors, C1 in Figure 26, are

typically 4 pF and can primarily be attributed to pin

capacitance. The resistors are lumped components made up of

the on resistance of the switches. The value of these resistors is

typically about 100 Ω. The capacitors C2 are the ADC’s

sampling capacitors and have a typical capacitance of 16 pF.

C1

C2

R1

D

D

C1

C2

R1

D

D

Figure 26. Equivalent Analog Input Circuit

Conversion Phase—Switches Open; Track Phase—Switches Closed

For ac applications, removing high frequency components from

the analog input signal through the use of an RC low-pass filter

on the relevant analog input pins is recommended. In applica-

tions where harmonic distortion and signal-to-noise ratio are

critical, the analog input should be driven from a low imped-

ance source. Large source impedances significantly affect the ac

performance of the ADC. This may necessitate the use of an

input buffer amplifier. The choice of the op amp is a function of

the particular application.

When no amplifier is used to drive the analog input, the source

impedance should be limited to low values. The maximum

source impedance depends on the amount of total harmonic

distortion (THD) that can be tolerated. The THD increases as

the source impedance increases, and performance degrades.

Figure 27 shows a graph of the THD versus the analog input

INPUT FREQUENCY (kHz)

10

100

–40

–60

–100

277

–20

0

–80

Figure 27. THD vs. Analog Input Frequency for Various

Figure 28 shows a graph of the THD vs. the analog input

sampling at 555 kSPS with an SCLK of 10 MHz. In this case, the

source impedance is 10 Ω.

10

100

–60

–65

–90

277

–55

–50

–70

–75

–80

–85

INPUT FREQUENCY (kHz)

Figure 28. THD vs. Analog Input Frequency for 3 V and 5 V Supply Voltages