Preliminary Technical Data

AD7357

Rev. PrF | Page 13 of 20

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

tolerated. 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.

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.

Figure 18. THD vs. Analog Input Frequency

ANALOG INPUTS

Differential signals have some benefits over single-ended

signals, including noise immunity based on the devices

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

common mode (CM).

The common mode 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,

conversion takes place, common mode is rejected resulting in a

corresponding to the digital codes of 0 to 16383 for the

AD7357.

DRIVING DIFFERENTIAL INPUTS

simultaneously 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 conversions.

Differential Amplifier

An ideal method of applying differential drive to the AD7357 is

be used as a single-ended-to-differential amplifier or as a

differential-to-differential amplifier. The AD8138 also provides

common-mode level shifting. Figure 20 shows how the AD8138

can be used as a single-ended-to-differential amplifier. The

positive and negative outputs of the AD8138 are connected to

the respective inputs on the ADC via a pair of series resistors to

minimize the effects of switched capacitance on the front end of

the ADC. The architecture of the AD8138 results in outputs

that are very highly balanced over a wide frequency range

without requiring tightly matched external components.