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AD7674 Datasheet(PDF) 18 Page - Analog Devices

Part No. AD7674
Description  18-Bit, 2.5 LSB INL, 800 kSPS SAR ADC
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Manufacturer  AD [Analog Devices]
Direct Link  http://www.analog.com
Logo AD - Analog Devices

AD7674 Datasheet(HTML) 18 Page - Analog Devices

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AD7674
Rev. A | Page 18 of 28
TYPICAL CONNECTION DIAGRAM
Figure 27 shows a typical connection diagram for the AD7674.
Different circuitry shown on this diagram is optional and is
discussed later in this data sheet.
Analog Inputs
Figure 28 shows a simplified analog input section of the
AD7674. The diodes shown in Figure 28 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 will cause these diodes to become forward biased
and start conducting current. These diodes can handle a
forward-biased current of 120 mA max. This condition could
eventually occur when the input buffer’s 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.
IN+
IN–
AGND
AVDD
R+ = 102
Ω
CS
CS
R– = 102
Ω
03083-0-028
Figure 28. Simplified Analog Input
This analog input structure is a true differential structure. By
using these differential inputs, signals common to both inputs
are rejected as shown in Figure 29, which represents typical
CMRR over frequency.
FREQUECY (kHz)
66
64
50
100
1000
10000
110
62
60
58
56
54
52
03083-0-029
Figure 29. Analog Input CMRR vs. Frequency
During the acquisition phase for ac signals, the AD7674
behaves like a 1-pole RC filter consisting of the equivalent
resistance R+, R–, and CS. The resistors R+ and R– are typically
102 Ω and are lumped components made up of a serial resistor
and the on resistance of the switches. CS is typically 60 pF and
mainly consists of the ADC sampling capacitor. This 1-pole
filter with a –3 dB cutoff frequency of 26 MHz typ reduces any
undesirable aliasing effect and limits the noise coming from the
inputs.
Because the input impedance of the AD7674 is very high, the
part can be driven directly by a low impedance source without
gain error. This allows the user to put an external 1-pole RC
filter between the amplifier output and the ADC analog inputs,
as shown in Figure 27, to improve the noise filtering done by
the AD7674 analog input circuit. However, the source
impedance has to be kept low because it affects the ac
performance, especially the total harmonic distortion (THD).
The maximum source impedance depends on the amount of
THD that can be tolerated. The THD degrades as a function of
source impedance and the maximum input frequency, as shown
in Figure 30.
INPUT RESISTANCE (
Ω)
–95
–120
45
75
105
15
–100
–105
–110
–115
20kHz
10kHz
2kHz
03083-0-030
Figure 30. THD vs. Analog Input Frequency and Source Resistance
Driver Amplifier Choice
Although the AD7674 is easy to drive, the driver amplifier
needs to meet the following requirements:
The driver amplifier and the AD7674 analog input circuit
have to 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% or 0.01% is more commonly
specified. This could differ significantly from the settling
time at an 18-bit level and, therefore, should be verified
prior to driver selection. The tiny op amp AD8021, which
combines ultralow noise and high gain-bandwidth, meets
this settling time requirement.
The noise generated by the driver amplifier needs to be
kept as low as possible in order to preserve the SNR and
transition noise performance of the AD7674. The noise
coming from the driver is filtered by the AD7674 analog
input circuit 1-pole low-pass filter made by R+, R–, and CS.
The SNR degradation due to the amplifier is


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