EVAL-AD7723CBZ Analog Devices Inc, EVAL-AD7723CBZ Datasheet - Page 18

• Manufacturer Part Number: EVAL-AD7723CBZ
• Description: BOARD EVALUATION FOR AD7723
• Manufacturer: Analog Devices Inc

Specifications of EVAL-AD7723CBZ

• Number Of Adc's: 1
• Number Of Bits: 16
• Sampling Rate (per Second): 1.2M
• Data Interface: Serial, Parallel
• Inputs Per Adc: 1 Differential
• Input Range: ±VREF
• Power (typ) @ Conditions: 475mW @ 1.2MSPS
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Utilized Ic / Part: AD7723
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD7723
CIRCUIT DESCRIPTION
The AD7723 ADC employs a Σ-Δ conversion technique to
convert the analog input into an equivalent digital word. The
modulator samples the input waveform and outputs an
equivalent digital word at the input clock frequency, f
Due to the high oversampling rate that spreads the quantization
noise from 0 to f
of interest is reduced (Figure 27A). To further reduce the
quantization noise, a high-order modulator is employed to
shape the noise spectrum so that most of the noise energy is
shifted out of the band of interest (Figure 27B).
The digital filter that follows the modulator removes the large
out-of-band quantization noise (Figure 27C) while also
reducing the data rate from f
f
state on the MODE1/MODE2 pins in parallel interface mode or
the SLDR pin in serial interface mode. The AD7723 output data
rate is a little over twice the signal bandwidth, which guarantees
that there is no loss of data in the signal band.
Digital filtering has certain advantages over analog filtering.
First, since digital filtering occurs after the A/D conversion, it
can remove noise injected during the conversion process.
Analog filtering cannot remove noise injected during
conversion. Second, the digital filter combines low pass-band
ripple with a steep roll-off while also maintaining a linear phase
response.
The AD7723 employs four or five finite impulse response (FIR)
filters in series. Each individual filter’s output data rate is half
that of the filter’s input data rate. When data is fed to the
interface from the output of the fourth filter, the output data
rate is f
CLKIN
/32 or f
CLKIN
BAND OF INTEREST
BAND OF INTEREST
BAND OF INTEREST
/16 and the resulting oversampling ratio (OSR) of
CLKIN
CLKIN
/16 at the output of the filter, depending on the
Figure 27. Sigma-Delta ADC
/2, the noise energy contained in the band
QUANTIZATION NOISE
DIGITAL FILTER CUTOFF FREQUENCY
NOISE SHAPING
CLKIN
A
B
C
at the input of the filter to
f
f
f
CLKIN
CLKIN
CLKIN
/2
/2
/2
CLKIN
.
Rev. C | Page 18 of 32
the converter is 16. Data fed to the interface from the output of
the fifth filter results in an output data rate of f
corresponding OSR for the converter of 32. When an output
data rate (ODR) of f
response can be set to either low-pass or band-pass. The band-
pass response is useful when the input signal is band limited
because the resulting output data rate is half that required to
convert the band when the low-pass operating mode is used. To
illustrate the operation of this mode, consider a band-limited
signal, as shown in Figure 28A. This signal band can be
correctly converted by selecting the (low-pass) ODR = f
mode, as shown in Figure 28B. Note that the output data rate is a
little over twice the maximum frequency in the frequency band.
Alternatively, the band-pass mode can be selected, as shown in
Figure 28C. The band-pass filter removes unwanted signals
from dc to just below f
f
f
maximum frequency of a little less than f
Figure 28D. Halving the output data rate reduces the workload
of any following signal processor and also allows a lower serial
clock rate to be used.
The frequency response of the three digital filter operating
modes is shown in Figure 29, Figure 30, and Figure 31.
CLKIN
CLKIN
/16, the output of the band-pass filter is sampled at
/32. This effectively translates the wanted band to a
0dB
0dB
0dB
0dB
LOW-PASS FILTER. OUTPUT DATA RATE =
LOW-PASS FILTER. OUTPUT DATA RATE = f
BAND-PASS FILTER
RESPONSE
FREQUENCY
TRANSLATED
INPUT SIGNAL
Figure 28. Band-Pass Operation
LOW-PASS FILTER RESPONSE
CLKIN
CLKIN
/32 is selected, the digital filter
BAND-PASS FILTER
BAND LIMITED SIGNAL
/64. Rather than outputting data at
ODR
A
B
C
D
SAMPLE
IMAGE
SAMPLE
IMAGE
SAMPLE
IMAGE
CLKIN
f
CLKIN
CLKIN
f
f
f
/64, as shown in
CLKIN
f
CLKIN
CLKIN
CLKIN
ODR
CLKIN
/16
/32
/16
/16
/16
/16
/32 and a
CLKIN
/16