AD7625BCPZ Datasheet, PDF (14/24 Pages) Analog Devices – 16-Bit, 6 MSPS, PulSAR Differential ADC

AD7625
TRANSFER FUNCTIONS
The AD7625 uses a 4.096 V reference. The AD7625 converts
the differential voltage of the antiphase analog inputs (IN+ and
IN−) into a digital output. The analog inputs, IN+ and IN−,
require a 2.048 V common-mode voltage (REF/2).
The 16-bit conversion result is in MSB first, twos complement
format.
The ideal transfer functions for the AD7625 are shown in
Figure 19 and Table 7.
011 ... 111
011 ... 110
011 ... 101
100 ... 010
100 ... 001
100 ... 000
–FSR
–FSR + 1LSB
–FSR + 0.5LSB
+FSR – 1LSB
+FSR – 1.5LSB
ANALOG INPUT
Figure 19. ADC Ideal Transfer Functions (FSR = Full-Scale Range)
Table 7. Output Codes and Ideal Input Voltages
Description
Analog Input
(IN+ − IN−)
REF = 4.096 V
Digital Output Code
Twos Complement (Hex)
FSR − 1 LSB
+4.0959375 V 0x1FFF
Midscale + 1 LSB +62.5 μV
0x0001
Midscale
0V
0x0000
Midscale − 1 LSB −62.5 μV
0xFFFF
−FSR + 1 LSB
−4.0959375 V 0x1001
−FSR
−4.096 V
0x1000
Data Sheet
ANALOG INPUTS
The analog inputs, IN+ and IN−, applied to the AD7625 must be
180° out of phase with each other. Figure 20 shows an equivalent
circuit of the input structure of the AD7625.
The two diodes provide ESD protection for the analog inputs,
IN+ and IN−. Care must be taken to ensure that the analog input
signal does not exceed the reference voltage by more than 0.3 V.
If the analog input signal exceeds this level, the diodes become
forward-biased and start conducting current. These diodes can
handle a forward-biased current of 130 mA maximum. However,
if the supplies of the input buffer (for example, the supplies of
the ADA4899-1 in Figure 24) are different from those of the
reference, the analog input signal may eventually exceed the
supply rails by more than 0.3 V. In such a case (for example, an
input buffer with a short circuit), the current limitation can be
used to protect the part.
VDD1
CNV
IN+
250Ω ^25pF
OR
IN–
Figure 20. Equivalent Analog Input Circuit
The analog input structure allows the sampling of the true
differential signal between IN+ and IN−. By using these differ-
ential inputs, signals common to both inputs are rejected. The
AD7625 shows some degradation in THD with higher analog
input frequencies.
85
80
75
70
65
60
55
50
45
40
1
10
100
1k
10k 100k
1M
10M
INPUT COMMON-MODE FREQUENCY (Hz)
Figure 21. Analog Input CMRR vs. Frequency
Rev. A | Page 14 of 24