AD7457BRT-R2 datasheet(12/20 Pages) AD

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AD7457

Rev. 0 | Page 12 of 20

VIN+

VIN–

VDD

SCLK

SDATA

GND

VREF

SERIAL

INTERFACE

+2.7V TO +5.25V

SUPPLY

2.5V

AD780

AD7457

VREF

P-TO-P

DC INPUT

VOLTAGE

µC/µP

0.1

µF

0.33

µF

Figure 17. Typical Connection Diagram

ANALOG INPUT

The AD7457 has a pseudo differential analog input. The VIN+

input is coupled to the signal source and should have an

amplitude of VREF p-p to make use of the full dynamic range of

the part. A dc input is applied to the VIN–. The voltage applied to

this input provides an offset from ground or a pseudo ground

for the VIN+ input. Ensure that (VIN− + VIN+) is less than or equal

to VDD to avoid exceeding the maximum ratings of the ADC.

The main benefit of pseudo differential inputs is that they

separate the analog input signal ground from the ADC’s ground,

allowing dc common-mode voltages to be canceled.

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

AD7457. See Figure 18.

When a conversion takes place, the pseudo ground corresponds

to 0 and the maximum analog input corresponds to 4096.

EXTERNAL

VREF (2.5V)

VIN+

VIN–

AD7457

2.5V

1.25V

VREF

+1.25V

–1

.25V

VIN

0.33

µF

Figure 18. Op Amp Configuration to Level Shift a Bipolar Input Signal

ANALOG INPUT STRUCTURE

Figure 19 shows the equivalent circuit of the analog input

structure of the AD7457. 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, which 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 19, 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 Ω. The capacitors, C2, are the ADC’s sampling capacitors,

which typically have a capacitance of 16 pF.

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

impedance source. Large source impedances can significantly

affect the ac performance of the ADC, which may necessitate

the use of an input buffer amplifier. The choice of the op amp is

a function of the particular application.

VIN+

VDD

VIN–

VDD

Figure 19. 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 20 shows a graph of the THD versus analog input signal

frequency for different source impedances.

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