adc12l032 National Semiconductor Corporation, adc12l032 Datasheet - Page 29

• Manufacturer Part Number: adc12l032
• Description: 3.3v Self-calibrating 12-bit Plus Sign Serial I/o A/d Converters With Mux And Sample/hold
• Manufacturer: National Semiconductor Corporation

Application Information
The reference voltage inputs are not fully differential. The
ADC12L030/2/4/8 will not generate correct conversions or
comparisons if V
sions result when V
at all times, between ground and V
mode range, (V
(0.6 x V
reference ladder should not go below 0.33V or above 1.98V.
Figure 15 is a graphic representation of the voltage restric-
tions on V
4.0 ANALOG INPUT VOLTAGE RANGE
The ADC12L030/2/4/8’s fully differential ADC generate a
two’s complement output that is found by using the equa-
tions shown below:
Round off to the nearest integer value between −4096 to
4095 for 12-bit resolution and between −256 to 255 for 8-bit
resolution if the result of the above equation is not a whole
number.
for (12-bit) resolution the Output Code =
for (8-bit) resolution the Output Code =
A
+
REF
). Therefore, with V
FIGURE 15. V
+
REF
and V
REF
+
REF
+
+ V
REF
is taken below V
+
REF
and V
−
REF
.
−
)/2, is restricted to (0.1 x V
Operating Range
REF
A
+
−
= 3.3V the center of the
differ by 1V and remain,
A
+
REF
. The V
−
(Continued)
. Correct conver-
REF
common
01183043
A
+
) to
29
Examples are shown in the table below:
5.0 INPUT CURRENT
At the start of the acquisition window (t
flows into or out of the analog input pins (A/DIN1 and
A/DIN2) depending on the input voltage polarity. The analog
input pins are CH0–CH7 and COM when A/DIN1 is tied to
MUXOUT1 and A/DIN2 is tied to MUXOUT2. The peak value
of this input current will depend on the actual input voltage
applied, the source impedance and the internal multiplexer
switch on resistance. With MUXOUT1 tied to A/DIN1 and
MUXOUT2 tied to A/DIN2 the internal multiplexer switch on
resistance is typically 1.6 kΩ. The A/DIN1 and A/DIN2 mux
on resistance is typically 750Ω.
6.0 INPUT SOURCE RESISTANCE
For low impedance voltage sources (
charging current will decay, before the end of the S/H’s
acquisition time of 2 µs (10 CCLK periods with f
to a value that will not introduce any conversion errors. For
high source impedances, the S/H’s acquisition time can be
increased to 18 or 34 CCLK periods. For less ADC resolution
and/or slower CCLK frequencies the S/H’s acquisition time
may be decreased to 6 CCLK periods. To determine the
number of clock periods (N
with a specific source impedance for the various resolutions
the following equations can be used:
Where f
and R
ample, operating with a resolution of 12 Bits+sign, a 5 MHz
clock frequency and maximum acquisition time of 34 conver-
sion clock periods the ADC’s analog inputs can handle a
source impedance as high as 6 kΩ. The acquisition time may
also be extended to compensate for the settling or response
time of external circuitry connected between the MUXOUT
and A/DIN pins.
An acquisition is started by a falling edge of SCLK and ends
with a rising edge of CCLK (see Timing Diagrams). If SCLK
and CCLK are asynchronous one extra CCLK clock period
may be inserted into the programmed acquisition time for
synchronization. Therefore, with asynchronous SCLK and
CCLK, the acquisition time will change from conversion to
conversion.
7.0 INPUT BYPASS CAPACITANCE
External capacitors (0.01 µF–0.1 µF) can be connected
between the analog input pins, CH0–CH7, and analog
ground to filter any noise caused by inductive pickup asso-
ciated with long input leads. These capacitors will not de-
grade the conversion accuracy.
+2.500V
+2.500V
+2.500V
12 Bit + Sign N
V
+2.5V
8 Bit + Sign N
REF
S
+
C
is the external source resistance in kΩ. As an ex-
is the conversion clock (CCLK) frequency in MHz
V
+1V
REF
0V
0V
0V
−
C
C
+2.499V +2.500V
+1.5V
= [R
= [R
V
+2V
0V
IN
+
S
S
c
) required for the acquisition time
+ 2.3] x f
+ 2.3] x f
+2.500V 1,0000,0000,0000
V
0V
0V
IN
−
C
C
x 0.824
x 0.57
A
) a charging current
0,1100,1100,1101
<
0,1111,1111,1111
1,1111,1111,1111
Digital Output
600Ω), the input
Code
C
www.national.com
= 5 MHz),