EVAL-AD7643CBZ Analog Devices Inc, EVAL-AD7643CBZ Datasheet - Page 18

• Manufacturer Part Number: EVAL-AD7643CBZ
• Description: BOARD EVALUATION FOR AD7643
• Manufacturer: Analog Devices Inc
• Series: PulSAR®r

Specifications of EVAL-AD7643CBZ

• Number Of Adc's: 1
• Number Of Bits: 18
• Sampling Rate (per Second): 1.25M
• Data Interface: Serial, Parallel
• Inputs Per Adc: 1 Differential
• Input Range: ±VREF
• Power (typ) @ Conditions: 65mW @ 1.25MSPS
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Utilized Ic / Part: AD7643
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD7643
DRIVER AMPLIFIER CHOICE
Although the AD7643 is easy to drive, the driver amplifier
needs to meet the following requirements:
•
•
•
For multichannel, multiplexed applications, the driver
amplifier and the AD7643 analog input circuit must be
able to settle for a full-scale step of the capacitor array at an
18-bit level (0.0004%). In the amplifier’s data sheet, settling
at 0.1% to 0.01% is more commonly specified. This could
differ significantly from the settling time at an 18-bit level
and should be verified prior to driver selection. The
AD8021
gain bandwidth, meets this settling time requirement even
when used with gains up to 13.
The noise generated by the driver amplifier needs to be
kept as low as possible to preserve the SNR and transition
noise performance of the AD7643. The noise coming from
the driver is filtered by the AD7643 analog input circuit
1-pole, low-pass filter made by R
external filter, if one is used. The SNR degradation due
to the amplifier is
where:
f
the cutoff frequency of the input RC filter shown in Figure 23
(3.9 MHz), if one is used.
N is the noise factor of the amplifier (1 in buffer
configuration).
e
of the op amps connected to IN+ and IN−, in nV/√Hz.
This approximation can be used when the resistances used
around the amplifier are small. If larger resistances are
used, their noise contributions should also be root-sum
squared.
The driver needs to have a THD performance suitable to
that of the AD7643. Figure 13 gives the THD vs. frequency
that the driver should exceed.
For instance, when using op amps with an equivalent input
noise density of 2.1 nV/√Hz, such as the AD8021, with a
noise gain of 1 when configured as a buffer, degrades the
SNR by only 0.25 dB when using the RC filter in Figure 23,
and by 2.5 dB without it.
SNR
–3dB
N+
and e
is the input bandwidth of the AD7643 (50 MHz) or
LOSS
N−
op amp, which combines ultralow noise and high
=
are the equivalent input voltage noise densities
20log
⎛
⎜
⎜
⎜
⎜ ⎜
⎝
900
+
π
f
−
2
3dB
IN
(
Ne
and C
N
30
+
)
2
IN
+
or by the
π
f
−
2
3dB
(
Ne
N
Rev. 0 | Page 18 of 28
−
)
2
⎞
⎟
⎟
⎟
⎟ ⎟
⎠
(UNIPOLAR 0V TO 2.048V)
The
almost all applications. The
compensation capacitor that should have good linearity as an
NPO ceramic or mica type. Moreover, the use of a noninverting
1 gain arrangement is recommended and helps to obtain the
best signal-to-noise ratio.
The
and a gain of 1 is present. The
applications where high frequency (above 100 kHz) performance is
not required. In applications with a gain of 1, an 82 pF
compensation capacitor is required. The
when low bias current is needed in low frequency applications.
Single-to-Differential Driver
For applications using unipolar analog signals, a single-ended-
to-differential driver, as shown in Figure 27, allows for a
differential input into the part. This configuration, when
provided an input signal of 0 to V
±V
and C = 2.7 nF provides a corner frequency of 3.9 MHz.
If the application can tolerate more noise, the
differential driver can be used.
VOLTAGE REFERENCE INPUT
The AD7643 allows the choice of either a very low temperature
drift internal voltage reference, an external 1.2 V reference that
can be buffered using the internal reference buffer, or an
external reference.
Unlike many ADCs with internal references, the internal
reference of the AD7643 provides excellent performance and
can be used in almost all applications.
REF
ANALOG INPUT
AD8021
AD8022
with midscale at V
5kΩ
5kΩ
Figure 27. Single-Ended-to-Differential Driver Circuit
meets these requirements and is appropriate for
can also be used when a dual version is needed
100nF
(Internal Reference Buffer Used)
U1
U2
590Ω
590Ω
REF
10pF
10pF
AD8021
AD8021
/2. The 1-pole filter using R = 15 Ω
AD8021
AD829
REF
2.7nF
2.7nF
15Ω
15Ω
, produces a differential
needs a 10 pF external
is an alternative in
AD8610
AD8139
is an option
IN+
IN–
AD7643
10µF
REF