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AD7357YRUZ Datasheet(PDF) 12 Page - Analog Devices

Part No. AD7357YRUZ
Description  Differential Input,Dual,Simultaneous Sampling, 4.25 MSPS, 14-Bit, SAR ADC
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD7357YRUZ Datasheet(HTML) 12 Page - Analog Devices

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AD7357
Preliminary Technical Data
Rev. PrD | Page 12 of 17
conduct up to 10mA without causing irreversible damage to the
part.
The C1 capacitors in Figure 11 are typically TBD pF and can
primarily be attributed to pin capacitance. The resistors are
lumped components made up of the on resistance of the
switches. The value of these resistors is typically about TBD Ω.
The C2 capacitors are the ADC’s sampling capacitors with a
capacitance of TBD pF typically.
VDD
C1
D
D
VIN+
R1 C2
VDD
C1
D
D
VIN–
R1 C2
Figure 11.Equivalent Analog Input Circuit,
Conversion Phase – Switches Open, Track Phase – Switches Closed
For ac applications, removing high frequency components from
the analog input signal is recommended by the use of an RC
low-pass filter on the analog input pins. In applications where
harmonic distortion and signal-to-noise ratio are critical, the
analog input should be driven from a low impedance source.
Large source impedances significantly affect the ac performance
of the ADC and may necessitate the use of an input buffer
amplifier. The choice of the op amp will be a function of the
particular application.
Figure 12.THD vs. Analog Input Frequency for Various Source Impedances
When no amplifier is used to drive the analog input, the source
impedance should be limited to low values. The maximum
source impedance will depend on the amount of THD that can
be tolerated. The THD increases as the source impedance
increases and performance degrades. Figure 12 shows a graph
of the THD vs. the analog input signal frequency for different
source impedances.
Figure 13 shows a graph of the THD vs. the analog input
frequency while sampling at 4.25 MSPS. In this case the source
impedance is TBD Ω.
Figure 13.THD vs. Analog Input Frequency
ANALOG INPUTS
Differential signals have some benefits over single-ended
signals, including noise immunity based on the devices
common-mode rejection and improvements in distortion
performance.
The amplitude of the differential signal is the difference
between the signals applied to the VIN+ and VIN- pins in each
differential pair (VIN+ - VIN-). VIN+ and VIN- should be
simultaneously driven by two signals each of amplitude VREF
that are 180° out of phase. This amplitude of the differential
signal is, therefore –VREF to +VREF peak-to –peak regardless of
the common mode (CM).
The common mode is the average of the two signals and is
therefore the voltage on which the two inputs are centered.
CM = (VIN+ + VIN-)/2
This results in the span of each input being CM ± VREF/2. This
voltage has to be set up externally. When a conversion takes
place, the common mode is rejected resulting in a virtually
noise free signal of amplitude –VREF to +VREF corresponding to
the digital codes of 0 to 16383 for the AD7357.


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