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| 型號： | AD7679CB1 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | 18-Bit, 2.5 LSB INL, 570 kSPS SAR ADC |
| 中文描述： | 18位，2.5 LSB INL和570 kSPS的SAR型ADC |
| 文件頁數(shù)： | 18/28頁 |
| 文件大小： | 506K |
| 代理商： | AD7679CB1 |
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AD7679
Rev. 0 | Page 18 of 28
U2
8.25k
2.5V
AD8021
10pF
590
AD7679
IN+
IN–REF
U1
ANALOG INPUT
(UNIPOLAR
0V TO 4.096V)
10pF
AD8021
590
10
μ
F
100nF
1.82k
2.7nF
30
2.7nF
30
REFBUFIN
03085-0-031
The SNR degradation due to the amplifier is
=
π
+
2
)
(
625
25
log
20
N
3dB
–
LOSS
Ne
f
SNR
where:
f
–
3dB
is the –3 dB input bandwidth in MHz of the AD7679
(26 MHz) or the cutoff frequency of the input filter, if used.
N
is the noise factor of the amplifiers (1 if in buffer
configuration).
e
N
is the equivalent input noise voltage of each op amp in
nV/
√
Hz.
Figure 29. Single-Ended-to-Differential Driver Circuit
(Internal Reference Buffer Used)
For instance, for a driver with an equivalent input noise of
2 nV/
√
Hz (e.g., AD8021) configured as a buffer, thus with a
noise gain of +1, the SNR degrades by only 0.34 dB with
the filter in Figure 25, and by 1.8 dB without it.
Voltage Reference
The AD7679 allows the use of an external voltage reference
either with or without the internal reference buffer.
Using the internal reference buffer is recommended when
sharing a common reference voltage between multiple ADCs is
desired.
The driver needs to have a THD performance suitable to
that of the AD7679.
The
AD8021
meets these requirements and is usually
appropriate for almost all applications. The AD8021 needs a
10 pF external compensation capacitor, which should have good
linearity as an NPO ceramic or mica type.
However, the advantages of using the external reference voltage
directly are
The SNR and dynamic range improvement (about 1.7 dB)
resulting from the use of a reference voltage very close to
the supply (5 V) instead of a typical 4.096 V reference when
the internal buffer is used.
The
AD8022
could be used if a dual version is needed and gain
of 1 is present. The
AD829
is an alternative in applications
where high frequency (above 100 kHz) performance is not
required. In gain of 1 applications, it requires an 82 pF
compensation capacitor. The
AD8610
is another option when
low bias current is needed in low frequency applications.
The power saving when the internal reference buffer is
powered down (PDBUF high).
To use the internal reference buffer, PDBUF should be LOW. A
2.5 V reference voltage applied on the REFBUFIN input will
result in a 4.096 V reference on the REF pin.
Single-to-Differential Driver
For applications using unipolar analog signals, a single-ended-
to-differential driver will allow for a differential input into the
part. The schematic is shown in Figure 29. When provided an
input signal of 0 to V
REF
, this configuration will produce a
differential ±V
REF
with midscale at V
REF
/2.
In both cases, the voltage reference input REF has a dynamic
input impedance and therefore requires an efficient decoupling
between REF and REFGND inputs. The decoupling consists of a
low ESR 47 μF tantalum capacitor connected to the REF and
REFGND inputs with minimum parasitic inductance.
If the application can tolerate more noise, the AD8138,
differential driver can be used.
Care should also be taken with the reference temperature
coefficient of the voltage reference, which directly affects the
full-scale accuracy if this parameter matters. For instance, a
±4 ppm/°C temperature coefficient of the reference changes the
full scale by ±1 LSB/°C.
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