ADC12020 Datasheet, PDF (17/29 Pages) National Semiconductor (TI) – 12-Bit, 20 MSPS, 185 mW A/D Converter with Internal Sample-and-Hold

ADC12020
www.ti.com
SNAS186B – APRIL 2003 – REVISED MARCH 2013
APPLICATIONS INFORMATION
OPERATING CONDITIONS
We recommend that the following conditions be observed for operation of the ADC12020:
• 4.75V ≤ V_A ≤ 5.25V
• V_D = V_A
• 2.35V ≤ V_DR ≤ V_D
• 100 kHz ≤ f_CLK ≤ 30 MHz
• 1.0V ≤ V_REF ≤ 2.4V
• 1.0V ≤ V_CM ≤ 4.0V
Analog Inputs
The ADC12020 has two analog signal inputs, V_IN+ and V_IN−. These two pins form a differential input pair. There
is one reference input pin, V_REF.
Reference Pins
The ADC12020 is designed to operate with a 2.0V reference, but performs well with reference voltages in the
range of 1.0V to 2.4V. Lower reference voltages will decrease the signal-to-noise ratio (SNR). Increasing the
reference voltage (and the input signal swing) beyond 2.4V will degrade THD for a full-scale input.
It is very important that all grounds associated with the reference voltage and the input signal make connection to
the ground plane at a single point to minimize the effects of noise currents in the ground path.
The three Reference Bypass Pins (V_RP, V_RM and V_RN) are made available for bypass purposes. These pins
should each be bypassed to ground with a 0.1 µF capacitor. Smaller capacitor values will allow faster recovery
from the power down mode, but may result in degraded noise performance. DO NOT LOAD these pins.
Signal Inputs
The signal inputs are V_IN+ and V_IN−. The input signal, V_IN, is defined as
V_IN = (V_IN+) – (V_IN−)
(3)
Figure 38 shows the expected input signal range.
Note that the common mode input voltage range is 1V to 3V with a nominal value of V_A/2. The input signals
should remain between ground and 4V.
The Peaks of the individual input signals (V_IN+ and V_IN−) should each never exceed the voltage described as
V_IN+, V_IN− = V_REF + V_CM
(4)
to maintain THD and SINAD performance.
Figure 38. Expected Input Signal Range
The ADC12020 performs best with a differential input with each input centered around V_CM. The peak-to-peak
voltage swing at both V_IN+ and V_IN− each 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.
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