EVAL-AD7677CBZ Analog Devices Inc, EVAL-AD7677CBZ Datasheet - Page 13

• Manufacturer Part Number: EVAL-AD7677CBZ
• Description: BOARD EVALUATION FOR AD7677
• Manufacturer: Analog Devices Inc
• Series: PulSAR®r

Specifications of EVAL-AD7677CBZ

• Number Of Adc's: 1
• Number Of Bits: 16
• Sampling Rate (per Second): 1M
• Data Interface: Serial, Parallel
• Inputs Per Adc: 1 Differential
• Input Range: ±VREF
• Power (typ) @ Conditions: 115mW @ 1MSPS
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Utilized Ic / Part: AD7677
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The diodes shown in Figure 6 provide ESD protection for the
inputs. Care must be taken to ensure that the analog input sig-
nal never exceeds the absolute ratings on these inputs. This will
cause these diodes to become forward-biased and start conduct-
ing current. These diodes can handle a forward-biased current
of 120 mA maximum. This condition could eventually occur
when the input buffer’s (U1) or (U2) supplies are different from
AVDD. In such case, an input buffer with a short-circuit current
limitation can be used to protect the part.
This analog input structure is a true differential structure. By
using these differential inputs, signals common to both inputs
are rejected as shown in Figure 7, which represents the typical
CMRR over frequency.
During the acquisition phase, for ac signals, the AD7677 behaves
like a one-pole RC filter consisting of the equivalent resis-
tance R+ , R–, and C
168 V and are lumped components made up of some serial
resistors and the on resistance of the switches. The capacitor C
typically 60 pF and is mainly the ADC sampling capacitor. This
one-pole filter with a typical –3 dB cutoff frequency of 15.8 MHz
reduces undesirable aliasing effect and limits the noise com-
ing from the inputs.
Because the input impedance of the AD7677 is very high, the
AD7677 can be driven directly by a low impedance source
without gain error. That allows the user to input, 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 AD7677 analog input
circuit. However, the source impedance has to be kept low
because it affects the ac performances, especially the total har-
monic distortion. The maximum source impedance depends
REV. A
Figure 7. Analog Input CMRR vs. Frequency
90
85
80
75
70
65
60
55
50
45
IN+
IN–
1k
Figure 6. Simplified Analog Input
10k
S
AGND
. The resistors R+ and R– are typically
FREQUENCY – Hz
AVDD
100k
R+ = 168
R– = 168
1M
C
C
S
S
10M
S
is
–13–
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 midscale at 1.25 V.
If the application can tolerate more noise, the AD8138 can
be used.
Driver Amplifier Choice
Although the AD7677 is easy to drive, the driver amplifier needs
to meet at least the following requirements:
• The driver amplifier and the AD7677 analog input circuit
• The noise generated by the driver amplifier needs to be kept
where
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
a 16-bit level and, therefore, it should 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 AD7677. The noise coming
from the driver is filtered by the AD7677 analog input circuit
one-pole, low-pass filter made by R+, R–, and C
degradation due to the amplifier is:
f
(15.8 MHz) or the cutoff frequency of the input filter if
any used.
N is the noise factor of the amplifiers (1 if in buffer con-
figuration).
e
nV/(Hz)
–3 dB
Figure 8. Single-Ended-to-Differential Driver Circuit
N
is the equivalent input noise voltage of each opamp in
2.5V REF
is the –3 dB input bandwidth in MHz of the AD7677
SNR
1/2
ANALOG INPUT
LOSS
.
(UNIPOLAR)
590
590
=
20
LOG
590
U1
U2






AD8021
AD8021
784
C
C
C
590
C
+
π
4
f
28
–
IN+
3
IN–
dB
AD7677
(
2.5V REF
N e
AD7677
REF
N
S
. The SNR
)
2






REF
,