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AD7357YRUZ Datasheet(PDF) 13 Page - Analog Devices

Part No. AD7357YRUZ
Description  Differential Input, Dual, Simultaneous Sampling, 4.2 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) 13 Page - Analog Devices

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AD7357
Rev. 0 | Page 13 of 20
For ac applications, it is recommended to remove high frequency
components from the analog input signal 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 is a function of the particular
application.
When no amplifier is used to drive the analog input, the source
impedance should be limited to low values. The maximum source
impedance depends on the amount of THD that can be tole-
rated. The THD increases as the source impedance increases
and performance degrades. Figure 17 shows a graph of the
THD vs. the analog input signal frequency for various source
impedances.
–89
–87
–85
–83
–81
–79
–77
–75
–73
–71
–69
–67
–65
100
200
1000
1500
2000
2500
FREQUENCY (kHz)
10
33
50
100
Figure 17. THD vs. Analog Input Frequency for Various Source Impedances
Figure 18 shows a graph of the THD vs. the analog input
frequency while sampling at 4.2 MSPS. In this case, the
source impedance is 33 Ω.
–90.0
–86.0
–82.0
–78.0
–74.0
–70.0
–66.0
0
1000
2000
3000
4000
5000
ANALOG INPUT FREQUENCY (kHz)
Figure 18. THD vs. Analog Input Frequency
ANALOG INPUTS
Differential signals have some benefits over single-ended
signals, including noise immunity based on the device’s
common-mode rejection and improvements in distortion
performance. Figure 19 defines the fully differential input
of the AD7357.
VIN+
AD7357*
VIN–
VREF p-p
VREF p-p
*ADDITIONAL PINS OMITTED FOR CLARITY.
COMMON
MODE
VOLTAGE
Figure 19. Differential Input Definition
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).
CM 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 setting up the CM,
ensure that that VIN+ and VIN− remain within GND/VDD. When
a conversion takes place, CM is rejected, resulting in a virtually
noise free signal of amplitude –VREF to +VREF corresponding to
the digital codes of 0 to 16,383.
DRIVING DIFFERENTIAL INPUTS
Differential operation requires VIN+ and VIN− to be driven simulta-
neously with two equal signals that are 180° out of phase. Because
not all applications have a signal preconditioned for differential
operation, there is often a need to perform a single-ended-to-
differential conversion.


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