AD7352 datasheet(14/21 Pages) AD

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Data Sheet

AD7352

Rev. B | Page 13 of 20

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 perfor-

mance 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, limit

the source impedance to low values. The maximum source

impedance depends on the amount of THD that can be

tolerated. THD increases as the source impedance increases

and performance degrades. Figure 17 shows a graph of THD

vs. the analog input signal frequency for different source

impedances.

–89

–87

–85

–83

–81

–79

–77

–75

–73

–71

–69

–67

–65

100

500

1000

1500

2000

2500

FREQUENCY (kHz)

100Ω

50Ω

33Ω

10Ω

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

Impedances

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

frequency while sampling at 3 MSPS. In this case, the source

impedance is 33 Ω.

–90

–86

–82

–78

–74

–70

–66

1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

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 devices

common-mode rejection and improvements in distortion

performance. Figure 19 defines the fully differential input of

the AD7352.

VIN+

AD7352*

VIN–

VREF p-p

*ADDITIONAL PINS OMITTED FOR CLARITY.

COMMON-MODE

VOLTAGE

Figure 19. Differential Input Definition

The amplitude of the differential signal is the difference

between the signals applied to the VIN+ and VIN− pins in

each differential pair (VIN+ − VIN−). VIN+ and VIN− should be

simultaneously driven by two signals each of amplitude (VREF)

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

signal is, therefore −VREF to +VREF peak-to-peak regardless of

the common mode (CM).

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

voltage on which the two inputs are centered.

CM = (VIN+ + VIN−)/2

This results in the span of each input being CM ± VREF/2. This

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

ensure that VIN+ and VIN− remain within GND/VDD. When

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

noise-free signal of amplitude, −VREF to +VREF, corresponding

to the digital codes of 0 to 4095 for the AD7352.

DRIVING DIFFERENTIAL INPUTS

Differential operation requires VIN+ and VIN− to be driven

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 conversion.

Differential Amplifier

An ideal method of applying differential drive to the AD7352

is to use a differential amplifier such as the AD8138. This part

can 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.

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