EVAL-AD7654CBZ Analog Devices Inc, EVAL-AD7654CBZ Datasheet - Page 16

• Manufacturer Part Number: EVAL-AD7654CBZ
• Description: BOARD EVALUATION FOR AD7654
• Manufacturer: Analog Devices Inc
• Series: PulSAR®r

Specifications of EVAL-AD7654CBZ

• Number Of Adc's: 2
• Number Of Bits: 16
• Sampling Rate (per Second): 500k
• Data Interface: Serial, Parallel
• Inputs Per Adc: 2 Differential
• Input Range: 0 ~ 5 V
• Power (typ) @ Conditions: 120mW @ 500kSPS
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Utilized Ic / Part: AD7654
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD7654
TYPICAL CONNECTION DIAGRAM
Figure 18 shows a typical connection diagram for the AD7654.
Different circuitry shown on this diagram is optional and is
discussed in the following sections.
ANALOG INPUTS
Figure 19 shows a simplified analog input section of the
AD7654.
The diodes shown in Figure 19 provide ESD protection for the
inputs. Care must be taken to ensure that the analog input
signal never exceeds the absolute ratings on these inputs. This
causes these diodes to become forward biased and start
conducting current. These diodes can handle a forward-biased
current of 120 mA maximum. This condition could eventually
occur when the input buffers (U1) or (U2) supplies are different
from AVDD. In such a case, an input buffer with a short-circuit
current limitation can be used to protect the part.
This analog input structure allows the sampling of the
differential signal between INx and INxN. Unlike other
converters, the INxN is sampled at the same time as the INx
input. By using these differential inputs, small signals common
to both inputs are rejected.
During the acquisition phase, for ac signals, the AD7654
behaves like a one-pole RC filter consisting of the equivalent
resistance R
500 Ω and are a lumped component made up of some serial
resistors and the on resistance of the switches. The capacitor C
is typically 32 pF and is mainly the ADC sampling capacitor.
This one-pole filter with a typical −3 dB cutoff frequency of
10 MHz reduces undesirable aliasing effects and limits the noise
coming from the inputs.
Because the input impedance of the AD7654 is very high, the
AD7654 can be driven directly by a low impedance source
without gain error. To further improve the noise filtering of the
AD7654 analog input circuit, an external one-pole RC filter
between the amplifier output and the ADC input, as shown in
Figure 18, can be used. However, the source impedance has to
be kept low because it affects the ac performance, especially the
total harmonic distortion. The maximum source impedance
depends on the amount of total harmonic distortion (THD)
INAN
INBN
INA2
INB2
INA1
INB1
A
, R
B
, and C
Figure 19. Simplified Analog Input
AGND
AVDD
S
. The resistors R
A0 = H
A0 = H
A0 = L
A0 = L
A0
A
and R
R
R
A
B
C
C
B
S
S
are typically
Rev. B | Page 16 of 28
S
• The noise generated by the driver amplifier needs to be kept
that can be tolerated. The THD degrades as the source
impedance increases.
INPUT CHANNEL MULTIPLEXER
The AD7654 allows the choice of simultaneously sampling the
inputs pairs INA1/INB1 or INA2/INB2 with the A0 multiplexer
input. When A0 is low, the input pairs INA1/INB1 are selected,
and when A0 is high, the input pairs INA2/INB2 are selected.
Note that INAx is always converted before INBx regardless of
the state of the digital interface channel selection A/ B pin. Also,
note that the channel selection control A0 should not be
changed during the acquisition phase of the converter. Refer to
the Conversion Control section and Figure 22 for timing details.
DRIVER AMPLIFIER CHOICE
Although the AD7654 is easy to drive, the driver amplifier
needs to meet at least the following requirements:
• For multichannel, multiplexed applications, the driver
• The driver needs to have a THD performance suitable to
amplifier and the AD7654 analog input circuit together
must be able 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.
as low as possible to preserve the SNR and transition noise
performance of the AD7654. The noise coming from the
driver is filtered by the AD7654 analog input circuit one-
pole low-pass filter made by R
degradation due to the amplifier is
where:
f
N
e
For instance, a driver like the AD8021 with an equivalent
input noise of 2 nV/√Hz, configured as a buffer, and thus
with a noise gain of +1, degrades the SNR by only 0.06 dB
with the filter in Figure 18, and by 0.10 dB without.
that of the AD7654.
–3 dB
N
is the –3 dB input bandwidth in MHz of the AD7654
(10 MHz) or the cutoff frequency of the input filter, if
any is used.
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.
SNR
LOSS
=
20
log
⎛
⎜
⎜
⎜
⎜
⎝
56
2
A
+
, R
π
2
B
f
56
, and C
−
3
dB
(
Ne
N
S
. The SNR
)
2
⎞
⎟
⎟
⎟
⎟
⎠