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參數資料
| 型號： | AD7676 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | 16-Bit +-1 LSB INL, 500 kSPS, Differential ADC |
| 中文描述： | 16位-1 LSB INL和500 kSPS的，差分ADC |
| 文件頁數： | 12/20頁 |
| 文件大小： | 321K |
| 代理商： | AD7676 |
REV. 0
AD7676
–
12
–
During the acquisition phase, for ac signals, the AD7676 behaves
like a one pole RC filter consisting of the equivalent resis-
tance R+ , R–,
and C
S
. The resistors R+ and R– are typically
684 V and are lumped components made up of some serial
resistors and the on-resistance of the switches. The capacitor
C
S
is typically 60 pF and is mainly the ADC sampling capacitor.
This one pole filter with a typical –3 dB cutoff frequency of
3.88 MHz reduces undesirable aliasing effect and limits the
noise coming from the inputs.
Because the input impedance of the AD7676 is very high, the
AD7676 can be driven directly by a low impedance source
without gain error. That allows users to put, as shown in
Figure 5, an external one-pole RC filter between the output
of the amplifier output and the ADC analog inputs to even
further improve the noise filtering done by the AD7676 analog
input circuit. However, the source impedance has to be kept
low because it affects the ac performances, especially the total
harmonic distortion. The maximum source impedance depends
on the amount of total harmonic distortion (THD) that can be
tolerated. The THD degrades proportionally to the source
impedance.
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 8.
U2
590
590
2.5V REF
C
C
AD8021
590
AD7676
IN+
IN
–
REF
2.5V REF
U1
ANALOG INPUT
(UNIPOLAR)
C
C
AD8021
590
Figure 8. Single-Ended to Differential Driver Circuit
This configuration, when provided an input signal of 0 to V
REF
,
will produce a differential
±
2.5 V with a common mode at 1.25 V.
If the application can tolerate more noise, the AD8138 can be used.
Driver Amplifier Choice
Although the AD7676 is easy to drive, the driver amplifier needs
to meet at least the following requirements:
The driver amplifier and the AD7676 analog input circuit
have to be able together to settle for a full-scale step of the
capacitor array at a 16-bit level (0.0015%). In the amplifier’s
data sheet, the settling at 0.1% or 0.01% is more commonly
specified. It could significantly differ from the settling time at
16-bit level and, therefore, it should be verified prior to the
driver selection. The tiny op amp AD8021 which combines
ultra low noise and a high gain bandwidth meets this settling
time requirement even when used with a high gain up to 13.
The noise generated by the driver amplifier needs to be kept as
low as possible in order to preserve the SNR and transition
noise performance of the AD7676. The noise coming from the
driver is filtered by the AD7676 analog input circuit one-pole,
low-pass filter made by R+, R–, and C
S
. The SNR degrada-
tion due to the amplifier is :
where
f
–3 dB
is the –3 dB input bandwidth of the AD7676 (3.88 MHz)
or the cutoff frequency of the input filter if any is used.
N
is the noise factor of the amplifier (1 if in buffer con-
figuration)
e
N
is the equivalent input noise voltage of the op amp in
nV/(Hz)
1/2
.
FSR
is 2
×
V
REF
.
For instance, a driver with an equivalent input noise of
2 nV/
√
Hz
like the AD8021 and configured as a buffer, thus with
a noise gain of +1, the SNR degrades by only 0.07 dB.
The driver needs to have a THD performance suitable to
that of the AD7676.
The AD8021 meets these requirements and is usually appropri-
ate for almost all applications. The AD8021 needs an external
compensation capacitor of 10 pF. This capacitor should have
good linearity as an NPO ceramic or mica type.
The AD8022 could also be used where dual version is needed
and gain of 1 is used.
The AD8132 or the AD8138 could also be used to generate a
differential signal from a single ended signal.
The AD829 is another alternative where high-frequency (above
500 kHz) performances is not required. In gain of 1, it requires
a 82 pF compensation capacitor.
The AD8610 is also another option where low-bias current is
needed in low-frequency applications.
Voltage Reference Input
The AD7676 uses an external 2.5 V voltage reference.
The voltage reference input REF of the AD7676 has a dynamic
input impedance. Therefore, it should be driven by a low-
impedance source with an efficient decoupling between REF
and REFGND inputs. This decoupling depends on the choice
of the voltage reference but usually consists of a low ESR
tantalum capacitor connected to the REF and REFGND
inputs with minimum parasitic inductance. 47
μ
F is an appro-
priate value for the tantalum capacitor when used with one of
the recommended reference voltages:
The low-noise, low temperature drift ADR421 and AD780
voltage references
The low-power ADR291 voltage reference
The low-cost AD1582 voltage reference
For applications using multiple AD7676s, it is more effective to
buffer the reference voltage with a low-noise, very stable op amp
like the AD8031.
SNR
LOG
f
N e
LOSS
dB
N
=
+
20
28
784
4
3
2
π
–
(
)
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| AD7676ACPZRL | 制造商:Analog Devices 功能描述:ADC Single SAR 500ksps 16-bit Parallel/Serial 48-Pin LFCSP EP T/R 制造商:Analog Devices 功能描述:ADC SGL SAR 500KSPS 16BIT PARALLEL/SERL 48LFCSP - Tape and Reel |
| AD7676AST | 制造商:Analog Devices 功能描述:ADC Single SAR 500ksps 16-bit Parallel/Serial 48-Pin LQFP 制造商:Analog Devices 功能描述:IC 16-BIT ADC |
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