EVAL-AD7401EDZ Analog Devices Inc, EVAL-AD7401EDZ Datasheet - Page 14

• Manufacturer Part Number: EVAL-AD7401EDZ
• Description: EVALUATION BOARD I.C.
• Manufacturer: Analog Devices Inc
• Series: iCoupler®r

Specifications of EVAL-AD7401EDZ

• Number Of Adc's: 1
• Number Of Bits: 16
• Sampling Rate (per Second): 20M
• Data Interface: Serial
• Inputs Per Adc: 1 Differential
• Input Range: ±320 mV
• Power (typ) @ Conditions: 100mW @ 20MSPS
• Voltage Supply Source: Single Supply
• Operating Temperature: -40°C ~ 105°C
• Utilized Ic / Part: AD7401
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD7401
DIFFERENTIAL INPUTS
The analog input to the modulator is a switched capacitor
design. The analog signal is converted into charge by highly
linear sampling capacitors. A simplified equivalent circuit
diagram of the analog input is shown in Figure 24. A signal
source driving the analog input must be able to provide the
charge onto the sampling capacitors every half MCLKIN cycle and
settle to the required accuracy within the next half cycle.
Because the AD7401 samples the differential voltage across its
analog inputs, low noise performance is attained with an input
circuit that provides low common-mode noise at each input.
The amplifiers used to drive the analog inputs play a critical role
in attaining the high performance available from the AD7401.
V
V
Figure 24. Analog Input Equivalent Circuit
IN
IN
+
–
1kΩ
1kΩ
MCLKIN
φA
φB
φA
φB
φA
φB
φA
φB
2pF
2pF
Rev. C | Page 14 of 20
When a capacitive load is switched onto the output of an op
amp, the amplitude momentarily drops. The op amp tries to
correct the situation and, in the process, hits its slew rate limit.
This nonlinear response, which can cause excessive ringing, can
lead to distortion. To remedy the situation, a low-pass RC filter
can be connected between the amplifier and the input to the
AD7401. The external capacitor at each input aids in supplying
the current spikes created during the sampling process, and the
resistor isolates the op amp from the transient nature of the load.
The recommended circuit configuration for driving the differ-
ential inputs to achieve best performance is shown in Figure 25. A
capacitor between the two input pins sources or sinks charge to
allow most of the charge that is needed by one input to be
effectively supplied by the other input. The series resistor again
isolates any op amp from the current spikes created during the
sampling process. Recommended values for the resistors and
capacitor are 22 Ω and 47 pF, respectively.
Figure 25. Differential Input RC Network
V
V
IN
IN
+
–
R
R
C
AD7401