Datasheet
ADC
Input
10 pF
V
CMO
0.1 PF
0.1 PF
V
IN
ETC1-1-13
0.1 PF
25:
25:
ETC1-1-13
18 pF
V
CMO
0.1 PF
0.1 PF
V
IN
ADT1-1WT
50:
0.1 PF
20:
20:
ADC
Input
ADC12C105
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SNAS417B –MAY 2007–REVISED AUGUST 2007
E
FS
= 4096 ( 1 - sin (90° + dev)) (5)
Where dev is the angular difference in degrees between the two signals having a 180° relative phase relationship
to each other (see Figure 31). For single frequency inputs, angular errors result in a reduction of the effective full
scale input. For complex waveforms, however, angular errors will result in distortion.
Figure 31. Angular Errors Between the Two Input Signals Will Reduce the Output Level or Cause
Distortion
It is recommended to drive the analog inputs with a source impedance less than 100Ω. Matching the source
impedance for the differential inputs will improve even ordered harmonic performance (particularly second
harmonic).
Table 1 indicates the input to output relationship of the ADC12C105.
Table 1. Input to Output Relationship
V
IN
+
V
IN
−
Binary Output 2’s Complement Output
V
CM
− V
REF
/2 V
CM
+ V
REF
/2 0000 0000 0000 1000 0000 0000 Negative Full-Scale
V
CM
− V
REF
/4 V
CM
+ V
REF
/4 0100 0000 0000 1100 0000 0000
V
CM
V
CM
1000 0000 0000 0000 0000 0000 Mid-Scale
V
CM
+ V
REF
/4 V
CM
− V
REF
/4 1100 0000 0000 0100 0000 0000
V
CM
+ V
REF
/2 V
CM
− V
REF
/2 1111 1111 1111 0111 1111 1111 Positive Full-Scale
Driving the Analog Inputs
The V
IN
+ and the V
IN
− inputs of the ADC12C105 have an internal sample-and-hold circuit which consists of an
analog switch followed by a switched-capacitor amplifier.
Figure 32 and Figure 32 show examples of single-ended to differential conversion circuits. The circuit in
Figure 32 works well for input frequencies up to approximately 70MHz, while the circuit in Figure 33 works well
above 70MHz.
Figure 32. Low Input Frequency Transformer Drive Circuit
Figure 33. High Input Frequency Transformer Drive Circuit
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