ADC

Input

10 pF

VCMO

VIN

ETC1-1-13

0.1 PF

25:

25:

ETC1-1-13

0.1 PF

0.1 PF

ADC

Input

18 pF

VCMO

0.1 PF

0.1 PF

VIN

ADT1-1WT

50:

0.1 PF

20:

20:

ADC12C080

www.ti.com

SNAS379C – SEPTEMBER 2006 – REVISED APRIL 2013

EFS = 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 100

Ω. 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.

Table 1. Input to Output Relationship

VIN+

VIN−

Binary Output

2’s Complement Output

VCM − VREF/2

VCM + VREF/2

0000 0000 0000

1000 0000 0000

Negative Full-Scale

VCM − VREF/4

VCM + VREF/4

0100 0000 0000

1100 0000 0000

VCM

VCM

1000 0000 0000

0000 0000 0000

Mid-Scale

VCM + VREF/4

VCM − VREF/4

1100 0000 0000

0100 0000 0000

VCM + VREF/2

VCM − VREF/2

1111 1111 1111

0111 1111 1111

Positive Full-Scale

Driving the Analog Inputs

The VIN+ and the VIN− 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.

Figure 32. Low Input Frequency Transformer Drive Circuit

Figure 33. High Input Frequency Transformer Drive Circuit

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