EVAL-AD7723CBZ Analog Devices Inc, EVAL-AD7723CBZ Datasheet - Page 21

• 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

With the unipolar input mode selected, just one op amp is
required to buffer single-ended input signals. However, driving
the AD7723 with complementary signals and with the bipolar
input range selected has some distinct advantages: even-order
harmonics in both the drive circuits and the AD7723 front end
are attenuated and the peak-to-peak input signal range on both
inputs is halved. Halving the input signal range allows some op
amps to be powered from the same supplies as the AD7723.
Although a complementary driver requires the use of two op
amps per ADC, it may avoid the need to generate additional
supplies just for these op amps.
Figure 38 and Figure 39 show two such circuits for driving the
AD7723. Figure 38 is intended for use when the input signal is
biased about 2.5 V, while Figure 39 is used when the input signal
is biased about ground. While both circuits convert the input
signal into a complementary signal, the circuit in Figure 39 also
level shifts the signal so that both outputs are biased about 2.5 V.
Suitable op amps include the AD8047, AD8044, AD8041, and
its dual equivalent, the AD8042. The AD8047 has lower input
noise than the AD8041/AD8042 but has to be supplied from a
+7.5 V/−2.5 V supply. The AD8041/AD8042 typically degrades
SNR from 90 dB to 88 dB but can be powered from the same
single 5 V supply as the AD7723.
ABOUT 2.5V
AIN = ±2V
Figure 38. Single-Ended-to-Differential Input Circuit for Bipolar Mode
BIASED
GAIN = 2 × R
R
R
R
Operation (Analog Input Biased About 2.5 V)
SOURCE
50Ω
10kΩ
FB
Figure 37. Input RC Network
390Ω
/(R
R
C
IN
SOURCE
220Ω
A2
A1
220Ω
220Ω
AD8047
AD8047
R
+ R
FB
220nF
IN
)
27Ω
27Ω
10nF
1µF
VIN(+)
VIN(–)
220pF
AD7723
20
19
23
21
VIN(+)
VIN(–)
REF2
REF1
AD7723
Rev. C | Page 21 of 32
APPLYING THE REFERENCE
The reference circuitry used in the AD7723 includes an on-chip
2.5 V band-gap reference and a reference buffer circuit. The
block diagram of the reference circuit is shown in Figure 40.
The internal reference voltage is connected to REF1 through a
3 kΩ resistor and is internally buffered to drive the analog
modulator’s switched cap DAC (REF2). When using the
internal reference, a 1 µF capacitor is required between REF1
and AGND to decouple the band-gap noise. If the internal
reference is required to bias external circuits, use an external
precision op amp to buffer REF1.
Where gain error or gain error drift requires the use of an
external reference, the reference buffer in Figure 40 can be
turned off by grounding the REF1 pin and the external
reference can be applied directly to REF2 pin. The AD7723
accepts an external reference voltage between 1.2 V to 3.15 V.
By applying a 3 V rather than a 2.5 V reference, SNR is typically
improved by about 1 dB. Where the output common-mode
range of the amplifier driving the inputs is restricted, the full-
scale input signal span can be reduced by applying a lower than
2.5 V reference. For example, a 1.25 V reference would make
the bipolar input span ±1 V but would degrade SNR.
GAIN = 2 × R
R
R
1µF
BALANCE1
REF2
Figure 39. Single-Ended-to-Differential Input Circuit for Bipolar Mode
= R
REF2
REF1
REF1
= R
AIN = ±2V
GROUND
FB
BIASED
ABOUT
/(R
BALANCE2
× (R
23
21
Operation (Analog Input Biased About Ground)
IN
IN
Figure 40. Reference Circuit Block Diagram
+ R
1V
+ R
SOURCE
SOURCE
× (R
R
R
10kΩ
SOURCE
REF1
IN
50Ω
R
R
20kΩ
BALANCE1
)
REF2
+ R
)/R
3kΩ
220Ω
COMPARATOR
SOURCE
FB
390Ω
R
IN
REFERENCE
)/(2 × R
R
R
BUFFER
REFERENCE
BALANCE2
BALANCE2
220Ω
A2
A1
220Ω
220Ω
AD8047
AD8047
R
2.5V
FB
FB
220nF
)
27Ω
27Ω
10nF
1µF
220pF
SWITCHED-CAP
DAC REFERENCED
20
19
23
21
AD7723
VIN(+)
VIN(–)
REF2
REF1
AD7723
AD7723