AD7357

Preliminary Technical Data

Rev. PrD | Page 12 of 17

conduct up to 10mA without causing irreversible damage to the

part.

The C1 capacitors in Figure 11 are typically TBD 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 TBD Ω.

The C2 capacitors are the ADC’s sampling capacitors with a

capacitance of TBD pF typically.

C1

D

D

R1 C2

C1

D

D

R1 C2

Figure 11.Equivalent Analog Input Circuit,

Conversion Phase – Switches Open, Track Phase – Switches Closed

For ac applications, removing high frequency components from

the analog input signal is recommended 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 will be a function of the

particular application.

Figure 12.THD vs. Analog Input Frequency for Various Source Impedances

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

impedance should be limited to low values. The maximum

source impedance will depend on the amount of THD that can

be tolerated. The THD increases as the source impedance

increases and performance degrades. Figure 12 shows a graph

of the THD vs. the analog input signal frequency for different

source impedances.

Figure 13 shows a graph of the THD vs. the analog input

frequency while sampling at 4.25 MSPS. In this case the source

impedance is TBD Ω.

Figure 13.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.

The amplitude of the differential signal is the difference

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

the 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 a conversion takes

place, the common mode is rejected resulting in a virtually

the digital codes of 0 to 16383 for the AD7357.