AD7357

Rev. 0 | Page 13 of 20

For ac applications, it is recommended to remove high frequency

components from the analog input signal by the use of an RC

low-pass filter on the analog input pins. 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 significantly affect the ac performance of the

ADC and 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 THD that can be tole-

rated. The THD increases as the source impedance increases

and performance degrades. Figure 17 shows a graph of the

THD vs. the analog input signal frequency for various source

impedances.

–89

–87

–85

–83

–81

–79

–77

–75

–73

–71

–69

–67

–65

100

200

1000

1500

2000

2500

FREQUENCY (kHz)

10

Ω

33

Ω

50

Ω

100

Ω

Figure 17. THD vs. Analog Input Frequency for Various Source Impedances

Figure 18 shows a graph of the THD vs. the analog input

frequency while sampling at 4.2 MSPS. In this case, the

source impedance is 33 Ω.

–90.0

–86.0

–82.0

–78.0

–74.0

–70.0

–66.0

0

1000

2000

3000

4000

5000

ANALOG INPUT FREQUENCY (kHz)

Figure 18. THD vs. Analog Input Frequency

ANALOG INPUTS

Differential signals have some benefits over single-ended

signals, including noise immunity based on the device’s

common-mode rejection and improvements in distortion

performance. Figure 19 defines the fully differential input

of the AD7357.

AD7357*

COMMON

MODE

VOLTAGE

Figure 19. Differential Input Definition

The amplitude of the differential signal is the difference

that are 180° out of phase. This amplitude of the differential

the common mode (CM).

CM is the average of the two signals and is, therefore, the

voltage on which the two inputs are centered.

voltage has to be set up externally. When setting up the CM,

a conversion takes place, CM is rejected, resulting in a virtually

the digital codes of 0 to 16,383.

DRIVING DIFFERENTIAL INPUTS

neously with two equal signals that are 180° out of phase. Because

not all applications have a signal preconditioned for differential

operation, there is often a need to perform a single-ended-to-

differential conversion.