ADC

Input

10 pF

VCMO

0.1 PF

0.1 PF

VIN

ETC1-1-13

0.1 PF

25:

25:

ETC1-1-13

ADC

Input

18 pF

VCMO

0.1 PF

0.1 PF

VIN

ADT1-1WT

50:

0.1 PF

20:

20:

ADC14C080

SNAS375C – SEPTEMBER 2006 – REVISED APRIL 2013

www.ti.com

For single frequency sine waves the full scale error in LSB can be described as approximately

EFS = 16384 ( 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 33). 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 33. 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 ADC14C080.

Table 1. Input to Output Relationship

VIN+

VIN−

Binary Output

2’s Complement Output

VCM − VREF/2

VCM + VREF/2

00 0000 0000 0000

10 0000 0000 0000

Negative Full-Scale

VCM − VREF/4

VCM + VREF/4

01 0000 0000 0000

11 0000 0000 0000

VCM

VCM

10 0000 0000 0000

00 0000 0000 0000

Mid-Scale

VCM + VREF/4

VCM − VREF/4

11 0000 0000 0000

01 0000 0000 0000

VCM + VREF/2

VCM − VREF/2

11 1111 1111 1111

01 1111 1111 1111

Positive Full-Scale

Driving the Analog Inputs

The VIN+ and the VIN− inputs of the ADC14C080 have an internal sample-and-hold circuit which consists of an

analog switch followed by a switched-capacitor amplifier.

Figure 34 and Figure 35 show examples of single-ended to differential conversion circuits. The circuit in

Figure 34 works well for input frequencies up to approximately 70MHz, while the circuit in Figure 35 works well

above 70MHz.

Figure 34. Low Input Frequency Transformer Drive Circuit

Figure 35. High Input Frequency Transformer Drive Circuit

20

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