AD7675 Analog Devices, AD7675 Datasheet - Page 12

• Manufacturer Part Number: AD7675
• Manufacturer: Analog Devices

Specifications of AD7675

• Resolution (bits): 16bit
• # Chan: 1
• Sample Rate: 100kSPS
• Interface: Par,Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: (2Vref) p-p
• Adc Architecture: SAR
• Pkg Type: QFP

During the acquisition phase for ac signals, the AD7675 behaves
like a one-pole RC filter consisting of the equivalent resistance
R+, R–, and C
and are lumped components made up of some serial resistors
and the on resistance of the switches. The capacitor C
60 pF and is mainly the ADC sampling capacitor. This one pole
filter with a typical –3 dB cutoff frequency of 3.9 MHz reduces
undesirable aliasing effect and limits the noise coming from
the inputs.
Because the input impedance of the AD7675 is very high, the
AD7675 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 AD7675 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.
This configuration, when provided an input signal of 0 to V
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 AD7675 is easy to drive, the driver amplifier needs
to meet at least the following requirements:
• The driver amplifier and the AD7675 analog input circuit
• The noise generated by the driver amplifier needs to be kept
AD7675
have to be able to settle for a full-scale step of the capaci-
tor 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 speci-
fied. 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.
as low as possible in order to preserve the SNR and transi-
tion noise performance of the AD7675. The noise coming
from the driver is filtered by the AD7675 analog input circuit
Figure 8. Single-Ended-to-Differential Driver Circuit
2.5V REF
ANALOG INPUT
(UNIPOLAR)
S
. The resistors R+ and R– are typically 684 Ω
590
590
590
U1
U2
AD8021
AD8021
C
C
C
C
590
IN+
IN–
AD7675
2.5V REF
REF
S
is typically
REF
,
–12–
where
For instance, in the case of 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 0.04 dB
with the filter in Figure 5, and 0.07 dB without.
• The driver needs to have a THD performance suitable to
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 differ-
ential signal from a single-ended signal. When using the AD8138
with the filter in Figure 5, the SNR degrades by only 0.9 dB.
The AD829 is another alternative where high frequency (above
100 kHz) performances are not required. In gain of 1, it requires
an 82 pF compensation capacitor.
The AD8610 is also another option where low bias current is
needed in low frequency applications.
The AD8519, OP162, or the OP184 could also be used.
Voltage Reference Input
The AD7675 uses an external 2.5 V voltage reference.
The voltage reference input REF of the AD7675 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 1 µF ceramic
capacitor and a low ESR tantalum capacitor connected to the
REF and REFGND inputs with minimum parasitic induc-
tance. 47 µF is an appropriate value for the tantalum capacitor
when used with one of the recommended reference voltages:
• The low noise, low temperature drift ADR421 and AD780
• The low power ADR291 voltage reference
• The low cost AD1582 voltage reference
For applications using multiple AD7675s, it is more effective to
buffer the reference voltage with a low noise, very stable op amp
like the AD8031.
one-pole, low-pass filter made by R+, R–, and C
degradation due to the amplifier is:
f
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
nV/(Hz)
that of the AD7675.
voltage references
–3 dB
N
is the equivalent input noise voltage of the op amp in
SNR
is the –3 dB input bandwidth of the AD7675 (3.9 MHz)
1/2
LOSS
.
=
20
LOG





784
+
4
π
f
28
–
3
dB
(
N e
S
N
. The SNR
)
2





REV. A