Rainbow Electronics ADC12020 - Manual (Page 17)

Functional Description
Operating on a single +5V supply, the ADC12020 uses a
pipeline architecture and has error correction circuitry to help
ensure maximum performance. The differential analog input
signal is digitized to 12 bits.
The reference input is buffered to ease the task of driving
that pin.
The output word rate is the same as the clock frequency,
which can be between 100 kSPS and 30 MSPS (typical).
The analog input voltage is acquired at the rising edge of the
clock and the digital data for a given sample is delayed by
the pipeline for 6 clock cycles.
A logic high on the power down (PD) pin reduces the con-
verter power consumption to 40 mW.
Applications Information
1.0 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
1.1 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
.
1.2 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) of the ADC12020. 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 analog 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.
1.3 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
−)
Figure 2 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 re-
main 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
to maintain THD and SINAD performance.
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.
For angular deviations of up to 10 degrees from these two
signals being 180 out of phase, the full scale error in LSB
can be described as approximately
E
FS
= dev 1.79
Where dev is the angular difference, in degrees, between
the two signals having a 180˚ relative phase relationship to
each other (see Figure 3). Drive the analog inputs with a
source impedance less than 100
Ω.
For differential operation, each analog input signal should
have a peak-to-peak voltage equal to the input reference
voltage, V
REF
, and be centered around a common mode
voltage, V
CM
.
TABLE 1. Input to Output Relationship —
Differential Input
V
IN+
V
IN−
Output
V
CM
− V
REF
/2 V
CM
+ V
REF
/2 0000 0000 0000
V
CM
− V
REF
/4 V
CM
+ V
REF
/4 0100 0000 0000
V
CM
V
CM
1000 0000 0000
V
CM
+ V
REF
/4 V
CM
− V
REF
/4 1100 0000 0000
V
CM
+ V
REF
/2 V
CM
− V
REF
/2 1111 1111 1111
20051711
FIGURE 2. Expected Input Signal Range
20051712
FIGURE 3. Angular Errors Between the Two Input
Signals Will Reduce the Output Level
ADC12020
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