AD7734 datasheet(29/32 Pages) AD

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AD7734

Rev. 0 | Page 29 of 32

Frequency Response

The sigma-delta modulator runs at ½ the MCLK frequency,

which is effectively the sampling frequency. Therefore, the

Nyquist frequency is ¼ the MCLK frequency. The digital filter,

in association with the modulator, features the frequency

response of a first order low-pass filter. The –3 dB point is close

to the frequency of 1/channel conversion time. The roll-off is

–20 dB/dec up to the Nyquist frequency. If chopping is enabled,

the input signal is resampled by chopping. Therefore, the overall

frequency response features notches close to the frequency of

1/channel conversion time. The top envelope is again the ADC

response of –20 dB/dec.

The typical frequency response plots are given in Figure 27

and Figure 28. The plots are normalized to 1/channel

conversion time.

NORMALIZED INPUT FREQUENCY

(INPUT FREQUENCY

× CONVERSION TIME)

–60

–50

–40

–30

–20

–10

0.1

1.0

10.0

CHOP = 1

Figure 27. Typical ADC Frequency Response, Chopping Enabled

NORMALIZED INPUT FREQUENCY

(INPUT FREQUENCY

× CONVERSION TIME)

–60

–50

–40

–30

–20

–10

0.1

1.0

10.0

CHOP = 1

Figure 28. Typical ADC Frequency Response, Chopping Disabled

Voltage Reference Inputs

The AD7734 has a differential reference input, REF IN(+) and

REF IN(–). The common-mode range for these inputs is from

AGND to AVDD. The nominal differential reference voltage for

specified operation is 2.5 V. Both reference inputs feature

dynamic load. Therefore, the reference inputs should be

connected to a low impedance reference voltage source.

External resistance/capacitance combinations may result in gain

errors on the part.

The output noise performance outlined in Table 4 through

Table 9 is for an analog input of 0 V and is unaffected by noise

on the reference. To obtain the same noise performance as

shown in the noise tables over the full input range requires a

low noise reference source for the AD7734. If the reference

noise in the bandwidth of interest is excessive, it will degrade

the performance of the AD7734.

Recommended reference voltage sources for the AD7734

include the AD780, ADR421, REF43, and REF192. Note that in a

typical connection, the voltage reference must be capable of

sinking current flowing out of the BIAS pins through the

internal resistors if a positive voltage is applied to the analog

input. The AD780 meets this requirement. If the voltage

reference used in an application is not capable of sinking

current, an external resistor (5 kΩ) should be connected in

parallel to the REFIN pins.

Reference Detect

The AD7734 includes on-chip circuitry to detect if the part has

a valid reference for conversions. If the voltage between the

REFIN(+) and REFIN(–) pins goes below the NOREF trigger

voltage (0.5 V typ.) and the AD7734 is performing a conversion,

the NOREF bit in the channel status register is set.

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