Datasheet

LM97593
SNWS019B –JULY 2007–REVISED APRIL 2013 www.ti.com
The nominal voltages for the reference bypass pins are as follows:
V
COM
= 1.5 V
V
RP
= V
COM
+ V
REF
/ 2
V
RN
= V
COM
− V
REF
/ 2
User choice of an on-chip or external reference voltage is provided. The internal 1.0 Volt reference is in use
when the the V
REF
pin is connected to V
A
. If a voltage in the range of 0.8V to 1.2V is applied to the V
REF
pin, that
is used for the voltage reference. When an external reference is used, the V
REF
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 V
REF
pin when
the internal reference is used.
Signal Inputs
The signal inputs are V
IN
A+ and V
IN
A− for one ADC and V
IN
B+ and V
IN
B− for the other ADC . The input signal,
V
IN
, is defined as
V
IN
A = (V
IN
A+) – (V
IN
A−) (1)
for the "A" converter and
V
IN
B = (V
IN
B+) – (V
IN
B−) (2)
for the "B" converter.
Figure 58 shows the expected input signal range. Note that the common mode input voltage, V
CM
, should be in
the range of 1.0V to 2.0V.
The peaks of the individual input signals should never exceed 2.6V.
The ADC performs best with a differential input signal with each input centered around a common mode voltage,
V
CM
. 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 58. Expected Input Signal Range
For single frequency sine waves the full scale error in LSBs can be described as approximately
E
FS
= 4096 ( 1 - sin (90° + dev))
where
• dev is the angular difference in degrees between the two signals having a 180° relative phase relationship to
each other (see Figure 59) (3)
Drive the analog inputs with a source impedance less than 100Ω.
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