AD7725 Analog Devices, AD7725 Datasheet - Page 15

• Manufacturer Part Number: AD7725
• Description: 16-Bit Sigma Delta ADC with a Programmable Post Processor
• Manufacturer: Analog Devices

Specifications of AD7725

• Resolution (bits): 16bit
• # Chan: 1
• Sample Rate: 14.4MSPS
• Interface: Par,Ser
• Analog Input Type: Diff-Bip,SE-Uni
• Ain Range: 4 V p-p,Uni 4.0V
• Adc Architecture: Sigma-Delta
• Pkg Type: QFP

CIRCUIT DESCRIPTION
The AD7725 employs a - 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, which spreads the quantiza-
tion noise from 0 to f
band of interest is reduced (Figure 9a). 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 9b).
The digital filtering that follows the modulator removes the
large out-of-band quantization noise (Figure 9c) while also
reducing the data rate from f
f
filter type used.
Digital filtering has certain advantages over analog filtering.
Because digital filtering occurs after the A/D conversion, it can
remove noise injected during the conversion process. Analog
filtering cannot do this. The digital filter also has a linear
phase response.
The AD7725 employs three fixed finite impulse response (FIR)
filters in series. Each individual filter’s output data rate is half
that of its input data rate. The fourth stage is programmable;
the user can select a range of different filter responses at this
stage. Both the filter response and the decimation are user pro-
grammable. See the Filtering section for more details.
REV. A
CLKIN
/16 or less at the output of the filter, depending on the
BAND OF INTEREST
BAND OF INTEREST
BAND OF INTEREST
CLKIN
Figure 9. - ADC
QUANTIZATION NOISE
DIGITAL FILTER CUTOFF FREQUENCY
/2, the noise energy contained in the
CLKIN
NOISE SHAPING
a.
b.
c.
at the input of the filter to
CLKIN
.
f
f
f
CLKIN
CLKIN
CLKIN
/2
/2
/2
–15–
APPLYING THE AD7725
Analog Input Range
The AD7725 has differential inputs to provide common-mode
noise rejection. In unipolar mode, the analog input is single-
ended and its range is 0 V to (8/5
the analog input is single-ended or differential, and its input
range is ± (4/5
in both modes with 1 LSB = 61 µV.
The ideal input/output transfer characteristics for the two
modes are shown in Figure 10. In both modes, the absolute
voltage on each input must remain within the supply range
AGND to AV
differential input signals while unipolar mode allows single-
ended signals.
The AD7725 will accept full-scale inband signals; however,
large scale out-of-band signals can overload the modulator inputs.
A minimal single-pole RC antialias filter set to f
full-scale input signals over the entire frequency spectrum.
Analog Input
The analog input of the AD7725 uses a switched capacitor
technique to sample the input signal. For the purpose of driving
the AD7725, an equivalent circuit of the analog inputs is shown
in Figure 11. For each half-clock cycle, two highly linear sam-
pling capacitors are switched to both inputs, converting the
input signal into an equivalent sampled charge. A signal source
driving the analog inputs must be able to source this charge,
while also settling to the required accuracy by the end of each
half-clock phase.
Figure 10. Bipolar/(Unipolar) Mode Transfer Function
011...111
011...110
000...010
000...001
000...000
111...111
111...110
100...001
100...000
–4/5
(0V)
DD
V
. Bipolar mode allows either single-ended or
DIFFERENTIAL INPUT VOLTAGE, V
REF2
V
REF2
). The output code is twos complement
(4/5
0V
V
REF2
V
)
REF2
(8/5
IN
4/5
). In bipolar mode,
(+) – V
CLKIN
V
V
REF2
REF2
AD7725
IN
(–)
/24 will allow
– 1LSB
– 1LSB)
BIPOLAR
UNIPOLAR