ADC12D040

SNAS171E – JUNE 2002 – REVISED MARCH 2013

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The VRN pins may be used as a common mode voltage source (VCM) for the analog input pins as long as no d.c.

current is drawn from it. However, because the voltages at these pins are half that of the VA supply pin, using

these pins for a common mode source will result in reduced input headroom (the difference between the VA

supply voltage and the peak signal voltage at either analog input) and the possibility of reduced THD and SFDR

performance. For this reason, it is recommended that VA always exceed VREF by at least 2 Volts. For high input

frequencies it may be necessary to increase this headroom to maintain THD and SFDR performance.

Signal Inputs

The signal inputs are VIN+ and VIN−. The input signal, VIN, is defined as

VIN = (VIN+) – (VIN−)

(6)

Figure 29 shows the expected input signal range.

Note that the common mode input voltage range is 1V to 3V with a nominal value of VA/2. The input signals

should remain between ground and 4V.

The Peaks of the individual input signals (VIN+ and VIN−) should each never exceed the voltage described as

VIN+, VIN− = (VREF / 2 + VCM) ≤ 4V (differential)

(7)

to maintain THD and SINAD performance.

Figure 29. Expected Input Signal Range

The ADC12D040 performs best with a differential input with each input centered around a common VCM. The

peak-to-peak voltage swing at both VIN+ and VIN− 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 a complex waveform,

however, angular errors will result in distortion.

For single frequency sine waves with angular errors of less than 45° (

π/4) between the two inputs, the full scale

error in LSB can be described as approximately

EFS = 2

(n-1) * ( 1 - cos (dev) ) = 2048 * ( 1 - cos (dev) )

(8)

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

other (see Figure 30). Drive the analog inputs with a source impedance less than 100

Ω.

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