AD7732 Analog Devices, AD7732 Datasheet - Page 29

• Manufacturer Part Number: AD7732
• Manufacturer: Analog Devices

Specifications of AD7732

• Resolution (bits): 24bit
• # Chan: 2
• Sample Rate: 3.05MSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Bip,Diff-Uni
• Ain Range: Bip 10V,Bip 5.0V,Uni 10V,Uni 5.0V
• Adc Architecture: Sigma-Delta
• Pkg Type: SOP

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.
–10
–20
–30
–40
–50
–60
–10
–20
–30
–40
–50
–60
Figure 28. Typical ADC Frequency Response, Chopping Disabled
Figure 27. Typical ADC Frequency Response, Chopping Enabled
0
0
0.1
0.1
(INPUT FREQUENCY  CONVERSION TIME)
(INPUT FREQUENCY  CONVERSION TIME)
CHOP = 1
1.0
NORMALIZED INPUT FREQUENCY
NORMALIZED INPUT FREQUENCY
CHOP = 0
10.0
1.0
100.0
1000.0
10.0
Rev. A | Page 29 of 32
Voltage Reference Inputs
The AD7732 has a differential reference input, REF IN(+) and
REF IN(–). The common-mode range for these inputs is from
AGND to AV
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 AD7732. If the reference
noise in the bandwidth of interest is excessive, it will degrade
the performance of the AD7732.
Recommended reference voltage sources for the AD7732
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 AD7732 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 AD7732 is performing a conversion,
the NOREF bit in the channel status register is set.
DD
. The nominal differential reference voltage for
AD7732