EVAL-AD7676CBZ Analog Devices Inc, EVAL-AD7676CBZ Datasheet - Page 12

• Manufacturer Part Number: EVAL-AD7676CBZ
• Description: BOARD EVALUATION FOR AD7676
• Manufacturer: Analog Devices Inc
• Series: PulSAR®r

Specifications of EVAL-AD7676CBZ

• Number Of Adc's: 1
• Number Of Bits: 16
• Sampling Rate (per Second): 500k
• Data Interface: Serial, Parallel
• Inputs Per Adc: 1 Differential
• Input Range: ±VREF
• Power (typ) @ Conditions: 67mW @ 500kSPS
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Utilized Ic / Part: AD7676
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD7676
During the acquisition phase for ac signals, the AD7676 behaves
like a one-pole RC filter consisting of the equivalent resistance
R+, R–, and C
are lumped components made up of some serial resistors and the
on resistance of the switches. The capacitor C
and is mainly the ADC sampling capacitor. This one-pole filter
with a typical –3 dB cutoff frequency of 3.88 MHz reduces unde-
sirable aliasing effects 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 (THD).
The maximum source impedance depends on the amount of
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 AD7676 is easy to drive, the driver amplifier needs
to meet the following requirements:
• The driver amplifier and the AD7676 analog input circuit have
• The driver needs to have a THD performance suitable to
• The noise generated by the driver amplifier needs to be kept
Figure 8. Single-Ended-to-Differential Driver Circuit
to be able, together, 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
specified. It could significantly differ from the settling time
at the 16-bit level and, therefore, it should be verified prior to
the driver selection. The tiny op amp AD8021, which com-
bines ultralow noise and a high gain bandwidth, meets this
settling time requirement even when used with a high gain
up to 13.
that of the AD7676.
as low as possible to preserve the SNR and transition noise
2.5V REF
ANALOG INPUT
S
(UNIPOLAR)
. The resistors R+ and R– are typically 684 Ω and
590
590
590
U1
U2
AD8021
AD8021
C
C
C
590
C
IN+
IN–
S
AD7676
is typically 60 pF
2.5V REF
REF
REF
,
–12–
The AD8021 meets these requirements and is usually appropriate
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 a dual version is needed
and a 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) performance is not required. In a 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.
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 imped-
ance source with an efficient decoupling between the 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 µF is an appropriate value for
the tantalum capacitor when used with one of the recommended
reference voltages:
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.
Care should also be taken with the reference temperature coeffi-
cient of the voltage reference, which directly affects the full-scale
accuracy if this parameter matters. For instance, a ± 15 ppm/°C
tempco of the reference changes the full scale by ± 1 LSB/°C.
• The low noise, low temperature drift ADR421 and AD780
• The low power ADR291 voltage reference
• The low cost AD1582 voltage reference
where:
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
configuration).
e
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, will degrade the SNR by only 0.26 dB.
–3 dB
N
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
the amplifier is:
voltage references
is the equivalent input noise voltage of the op amp in nV/√Hz.
SNR
is the –3 dB input bandwidth of the AD7676 (3.9 MHz)
LOSS
=
20
LOG




784
S
. The SNR degradation due to
+
π
f
−
28
3
dB
(
N e
N
)
2
REV. B



