REV. 0

–11–

AD7453

THE ANALOG INPUT

input is coupled to the signal source and must have an ampli-

this input provides an offset from ground or a pseudo ground

is that they separate the analog input signal ground from the

ADC’s ground, allowing dc common-mode voltages to be

cancelled.

Because the ADC operates from a single supply, it is necessary

to level shift ground based bipolar signals to comply with the

input requirements. An op amp (for example, the AD8021) can

be configured to rescale and level shift a ground based (bipolar)

signal so that it is compatible with the input range of the AD7453.

See Figure 7.

When a conversion takes place, the pseudo ground corresponds

to 0 and the maximum analog input corresponds to 4096.

EXTERNAL

R

AD7453

5V

2.5V

0V

+2.5V

0V

–2.5V

R

R

0.1 F

R

Figure 7. Op Amp Configuration to Level Shift a

Bipolar Input Signal

Analog Input Structure

Figure 8 shows the equivalent circuit of the analog input structure

of the AD7453. 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 will cause 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 8, are typically 4 pF and can be

attributed primarily 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

W. The capacitors,

C2, are the ADC’s sampling capacitors and have a capacitance

of 16 pF typically.

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 the signal-to-noise ratio are

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

ance source. Large source impedances will significantly affect

the ac performance of the ADC, which may necessitate the use

of an input buffer amplifier. The choice of the op amp will be a

function of the particular application.

C1

C2

R1

D

D

C1

C2

R1

D

D

Figure 8. Equivalent Analog Input Circuit. Conversion

Phase—Switches Open; Track Phase—Switches Closed

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 9 shows a

graph of the THD versus analog input signal frequency for

different source impedances.

INPUT FREQUENCY (kHz)

–100

10

100

277

–80

–50

–40

0

200

–10

–20

–30

–60

–70

–90

10

62

100

Figure 9. THD vs. Analog Input Frequency for

Various Source Impedances