AD7671AST Analog Devices Inc, AD7671AST Datasheet - Page 15

• Manufacturer Part Number: AD7671AST
• Description: IC ADC 16BIT CMOS 1MSPS 48-LQFP
• Manufacturer: Analog Devices Inc
• Series: PulSAR®r

Specifications of AD7671AST

• Rohs Status: RoHS non-compliant
• Number Of Bits: 16
• Sampling Rate (per Second): 1M
• Data Interface: Serial, Parallel
• Number Of Converters: 1
• Power Dissipation (max): 125mW
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 48-LQFP
• For Use With: EVAL-AD7671CBZ - BOARD EVALUATION FOR AD7671

Driver Amplifier Choice
Although the AD7671 is easy to drive, the driver amplifier needs
to meet at least the following requirements:
∑ The driver amplifier and the AD7671 analog input circuit
∑ The noise generated by the driver amplifier needs to be kept
∑ The driver needs to have a THD performance suitable to that
The AD8021 meets these requirements and is usually appropriate
for almost all applications. The AD8021 needs an external com-
pensation capacitor of 10 pF. This capacitor should have good
linearity as an NPO ceramic or mica type.
The AD8022 could also be used where a dual version is needed
and a gain of 1 is used.
The AD829 is another alternative where high frequency (above
100 kHz) performance is not required. In a gain of 1, it requires
an 82 pF compensation capacitor.
The AD8610 is another option where low bias current is needed
in low frequency applications.
REV. B
must be able, together, to settle for a full-scale step the capaci-
tor array at a 16-bit level (0.0015%). In the amplifier’s data
sheet, the settling at 0.1% to 0.01% is more commonly speci-
fied. It could significantly differ from the settling time at
16-bit level and it should therefore be verified prior to the
driver selection. The tiny op amp AD8021, which combines
ultralow 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 AD7671. The noise coming
from the driver is first scaled down by the resistive scaler
according to the analog input voltage range used and is then
filtered by the AD7671 analog input circuit one-pole, low-
pass filter made by (R/2 + R1) and C
due to the amplifier is
where:
f
N
e
FSR is the full-scale span (i.e., 5 V for ± 2.5 V range).
For instance, when using the 0 V to 5 V range, a driver like
the AD8021, with an equivalent input noise of 2 nV/ Hz and
configured as a buffer, thus with a noise gain of 1, the SNR
degrades by only 0.08 dB.
of the AD7671. TPC 11 gives the THD versus frequency
that the driver should preferably exceed.
–3 dB
N
SNR
is the –3 dB input bandwidth in MHz of the AD7671
(9.6 MHz) or the cutoff frequency of the input filter if
any used (0 V to 2.5 V range).
is the noise factor of the amplifier (1 if in buffer
configuration).
is the equivalent input noise voltage of the op amp
in nV/ Hz.
LOSS
=
20
LOG
Ê
Á
Á
Á
Á
Á
Ë
784
+
p
2
S
f
. The SNR degradation
–
3
28
dB
Ê
Á
Ë
2 5
.
FSR
N e
N
ˆ
˜
¯
2
ˆ
˜
˜
˜
˜
˜
¯
–15–
Voltage Reference Input
The AD7671 uses an external 2.5 V voltage reference.
The voltage reference input REF of the AD7671 has a dynamic
input impedance; it should therefore 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 mF ceramic capacitor and a
low ESR tantalum capacitor connected to the REF and REFGND
inputs with minimum parasitic inductance. 47 mF 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 AD7671s, it is more effective to
buffer the reference voltage with a low noise, very stable op amp
like the AD8031.
Care should also be taken with the reference temperature coeffi-
cient of the voltage reference that directly affects the full-scale
accuracy if this parameter matters. For instance, a ± 15 ppm/∞C
temperature coefficient of the reference changes the full scale
by ± 1 LSB/∞C.
Note that V
be increased to AVDD – 1.85 V. The benefit here is the increased
SNR obtained as a result of this increase. Since the input range
is defined in terms of V
± REF range from ± 2.5 V to ± 3 V and so on with an AVDD
above 4.85 V. The theoretical improvement as a result of this
increase in reference is 1.58 dB (20 log [3/2.5]). Due to the
theoretical quantization noise, however, the observed improve-
ment is approximately 1 dB. The AD780 can be selected with a
3 V reference voltage.
Scaler Reference Input (Bipolar Input Ranges)
When using the AD7671 with bipolar input ranges, the connec-
tion diagram in Figure 5 shows a reference buffer amplifier. This
buffer amplifier is required to isolate the REF pin from the signal
dependent current in the INx pin. A high speed op amp, such as the
AD8031, can be used with a single 5 V power supply without
degrading the performance of the AD7671. The buffer must have
good settling characteristics and provide low total noise within
the input bandwidth of the AD7671.
Power Supply
The AD7671 uses three sets of power supply pins: an analog 5 V
supply AVDD, a digital 5 V core supply DVDD, and a digital
input/output interface supply OVDD. The OVDD supply allows
direct interface with any logic working between 2.7 V and DVDD
+ 0.3 V. To reduce the number of supplies needed, the digital core
(DVDD) can be supplied through a simple RC filter from the
analog supply as shown in Figure 5. The AD7671 is independent
of power supply sequencing, once OVDD does not exceed DVDD
by more than 0.3 V, and thus free from supply voltage induced
latch-up. Additionally, it is very insensitive to power supply varia-
tions over a wide frequency range as shown in Figure 9.
voltage references
REF
, as mentioned in the Specifications table, could
REF
, this would essentially increase the
AD7671