EVAL-AD7961FMCZ View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
'EVAL-AD7961FMCZ' PDF : 24 Pages View PDF
Data Sheet
TRANSFER FUNCTION
The AD7961 uses a 5 V or a 4.096 V reference. The AD7961
converts the differential voltage of the antiphase analog inputs
(IN+ and IN−) into a digital output. IN+ and IN− require a
REF/2 V common-mode voltage.
The 16-bit conversion result is in MSB first, twos complement
format. The ideal transfer functions for the AD7961 are shown
in Figure 29 and Table 8.
AD7961
maximum. However, if the supplies of the input buffer amplifier
are different from the VDD1/GND supply, 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
IN+
OR IN–
185Ω 26pF
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 29. ADC Ideal Transfer Functions (FSR = Full-Scale Range)
ANALOG INPUTS
The analog inputs applied to the AD7961, IN+ and IN−, must be
180° out of phase with each other. Figure 30 shows an equivalent
circuit of the input structure of the AD7961.
The two diodes provide ESD protection for IN+ and IN−. Care
must be taken to ensure that the analog input signals do not exceed
the supply rails of the AD7961 by more than 0.3 V (VDD1 and
GND). If the analog input signals exceed this level, the diodes
become forward-biased and start conducting current. These
diodes can handle a forward-biased current of 130 mA
Figure 30. 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
AD7961 shows some degradation in THD with higher analog
input frequencies.
100
90
80
70
60
50
40
30
20
10
0
100
1k
10k
100k
1M
FREQUENCY (Hz)
Figure 31. Analog Input CMRR vs. Frequency
Table 8. Output Codes and Ideal Input Voltages
Description
Analog Input (IN+ − IN−), Analog Input (IN+ − IN−),
REF = 5 V
REF = 4.096 V
FSR − 1 LSB
+4.999847 V
+4.095875 V
Midscale + 1 LSB +152.6 μV
+125 μV
Midscale
0V
0V
Midscale − 1 LSB −152.6 μV
−125 μV
−FSR + 1 LSB
−4.999847 V
−4.095875 V
−FSR
−5 V
−4.096 V
Digital Output Code, Twos Complement (Hex)
0x7FFF
0x0001
0x0000
0xFFFF
0x8001
0x8000
Rev. B | Page 15 of 24
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