ADC12C080 Datasheet, PDF (21/32 Pages) National Semiconductor (TI) – 12-Bit, 65/80 MSPS A/D Converter

ADC12C080
www.ti.com
SNAS379C – SEPTEMBER 2006 – REVISED APRIL 2013
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 30). 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 ADC12C080.
V_IN⁺
V_CM − V_REF/2
V_CM − V_REF/4
V_CM
V_CM + V_REF/4
V_CM + V_REF/2
Table 1. Input to Output Relationship
V_IN^−
V_CM + V_REF/2
V_CM + V_REF/4
V_CM
V_CM − V_REF/4
V_CM − V_REF/2
Binary Output
0000 0000 0000
0100 0000 0000
1000 0000 0000
1100 0000 0000
1111 1111 1111
2’s Complement Output
1000 0000 0000
1100 0000 0000
0000 0000 0000
0100 0000 0000
0111 1111 1111
Negative Full-Scale
Mid-Scale
Positive Full-Scale
Driving the Analog Inputs
The V_IN+ and the V_IN− inputs of the ADC12C080 have an internal sample-and-hold circuit which consists of an
analog switch followed by a switched-capacitor amplifier.
Figure 32 and Figure 33 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.
V_{IN 0.1 PF ADT1-1WT}
20:
50:
0.1 PF
18 pF
0.1 PF
20:
V_CMO
ADC
Input
Figure 32. Low Input Frequency Transformer Drive Circuit
V_{IN 0.1 PF ETC1-1-13}
25:
0.1 PF
25:
ETC1-1-13
10 pF
ADC
Input
0.1 PF
V_CMO
Figure 33. High Input Frequency Transformer Drive Circuit
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