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參數(shù)資料
| 型號： | EVAL-AD7723CBZ |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 14/32頁 |
| 文件大?。?/td> | 0K |
| 描述： | BOARD EVALUATION FOR AD7723 |
| 標準包裝： | 1 |
| ADC 的數(shù)量： | 1 |
| 位數(shù)： | 16 |
| 采樣率（每秒）： | 1.2M |
| 數(shù)據(jù)接口： | 串行，并聯(lián) |
| 輸入范圍： | ±VREF |
| 在以下條件下的電源（標準）： | 475mW @ 1.2MSPS |
| 工作溫度： | -40°C ~ 85°C |
| 已用 IC / 零件： | AD7723 |
| 已供物品： | 板，CD |
| 相關(guān)產(chǎn)品： | AD7723BSZ-REEL-ND - IC ADC 16BIT SIGMA-DELTA 44MQFP AD7723BSZ-ND - IC ADC 16BIT SIGMA-DELTA 44MQFP |
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AD7723
Rev. C | Page 21 of 32
VIN(+)
VIN(–)
AD7723
R
C
01186-037
Figure 37. Input RC Network
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.
AD8047
RFB
220
220
27
220pF
A1
A2
VIN(+)
VIN(–)
REF1
REF2
AD7723
10nF
220nF
220
27
AIN =
±2V
BIASED
ABOUT 2.5V
1
F
RSOURCE
50
RIN
390
GAIN = 2
× R
FB/(RSOURCE + RIN)
10k
01186-038
21
23
19
20
Figure 38. Single-Ended-to-Differential Input Circuit for Bipolar Mode
Operation (Analog Input Biased About 2.5 V)
AD8047
RFB
220
220
27
220pF
A1
A2
VIN(+)
VIN(–)
REF1
REF2
AD7723
10nF
220nF
220
27
AIN =
±2V
BIASED
ABOUT
GROUND
1
F
RSOURCE
50
RIN
390
RBALANCE1
220
GAIN = 2
× R
FB/(RIN + RSOURCE)
RBALANCE1 = RBALANCE2 × (RIN + RSOURCE)/(2 × RFB)
RREF2 = RREF1 × (RIN + RSOURCE)/RFB
RREF1
10k
RREF2
20k
RBALANCE2
01186-
039
21
23
19
20
Figure 39. Single-Ended-to-Differential Input Circuit for Bipolar Mode
Operation (Analog Input Biased About Ground)
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 k resistor and is internally buffered to drive the analog
modulator’s switched cap DAC (REF2). When using the
internal reference, a 1 F 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.
COMPARATOR
REFERENCE
BUFFER
1V
1
F
REF2
REF1
2.5V
REFERENCE
SWITCHED-CAP
DAC REFERENCED
AD7723
3k
01186-040
23
21
Figure 40. Reference Circuit Block Diagram
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.
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