ADC12DL040

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SNAS250D – FEBRUARY 2005 – REVISED APRIL 2013

User choice of an on-chip or external reference voltage is provided. The internal 1.0 Volt reference is in use

when the the VREF pin is connected to VA. When the VREF pin is connected to AGND, the internal 0.5 Volt

reference is in use. If a voltage in the range of 0.8V to 1.2V is applied to the VREF pin, that is used for the voltage

reference. When an external reference is used, the VREF pin should be bypassed to ground with a 0.1 µF

capacitor close to the reference input pin. There is no need to bypass the VREF pin when the internal reference is

used.

Signal Inputs

The signal inputs are VIN A+ and VINA− for one ADC and VINB+ and VINB− for the other ADC . The input signal,

VIN, is defined as

VIN A = (VINA+) – (VINA−)

(5)

for the "A" converter and

VIN B = (VINB+) – (VINB−)

(6)

for the "B" converter. Figure 35 shows the expected input signal range. Note that the common mode input

voltage, VCM, should be in the range of 0.5V to 2.0V.

The peaks of the individual input signals should each never exceed 2.6V.

The ADC12DL040 performs best with a differential input signal with each input centered around a common mode

voltage, VCM. The peak-to-peak voltage swing at each analog input pin should not exceed the value of the

reference voltage or the output data will be clipped.

The two input signals should be exactly 180° out of phase from each other and of the same amplitude. 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 35. Expected Input Signal Range

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

EFS = 4096 ( 1 - sin (90° + dev))

(7)

Where dev is the angular difference in degrees between the two signals having a 180° relative phase relationship

to each other (see Figure 36). Drive the analog inputs with a source impedance less than 100

Ω.

Figure 36. Angular Errors Between the Two Input Signals Will Reduce the Output Level or Cause

Distortion

For differential operation, each analog input pin of the differential pair should have a peak-to-peak voltage equal

to the reference voltage, VREF, be 180 degrees out of phase with each other and be centered around VCM.

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