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ADC12H030 Datasheet(PDF) 34 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Part No. ADC12H030
Description  Self-Calibrating 12-Bit Plus Sign Serial I/O A/D Converters with MUX and Sample/Hold
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Maker  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com
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ADC12H030 Datasheet(HTML) 34 Page - National Semiconductor (TI)

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Application Hints (Continued)
4.0 ANALOG INPUT VOLTAGE RANGE
The ADC12030/2/4/8’s fully differential ADC generate a
two’s complement output that is found by using the equa-
tions shown below:
for (12-bit) resolution the Output Code =
for (8-bit) resolution the Output Code =
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.
Examples are shown in the table below:
Digital
V
REF
+
V
REF
V
IN
+
V
IN
Output
Code
+2.5V
+1V
+1.5V
0V
0,1111,1111,1111
+4.096V
0V
+3V
0V
0,1011,1011,1000
+4.096V
0V
+2.499V +2.500V
1,1111,1111,1111
+4.096V
0V
0V
+4.096V 1,0000,0000,0000
5.0 INPUT CURRENT
At the start of the acquisition window (t
A) a charging current
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 k
Ω. The A/DIN1 and A/DIN2 mux
on resistance is typically 750
Ω.
6.0 INPUT SOURCE RESISTANCE
For low impedance voltage sources (<600
Ω), the input
charging current will decay, before the end of the S/H’s
acquisition time of 2 µs (10 CCLK periods with f
C = 5 MHz),
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
c) required for the acquisition time
with a specific source impedance for the various resolutions
the following equations can be used:
12 Bit + Sign
N
C =[RS + 2.3] x fC x 0.824
8 Bit + Sign
N
C =[RS + 2.3] x fC x 0.57
Where f
C is the conversion clock (CCLK) frequency in MHz
and R
S is the external source resistance in k
Ω. Asanex-
ample, operating with a resolution of 12 Bits+sign,a5MHz
clock frequency and maximum acquistion time of 34 conver-
sion clock periods the ADC’s analog inputs can handle a
source impedance as high as 6 k
Ω. 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.
The acquisition time t
A is started by a falling edge of SCLK
and ended by 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 asnychronous SCLK and
CCLKs 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.
8.0 NOISE
The leads to each of the analog multiplexer input pins should
be kept as short as possible. This will minimize input noise
and clock frequency coupling that can cause conversion
errors. Input filtering can be used to reduce the effects of the
noise sources.
9.0 POWER SUPPLIES
Noise spikes on the V
A
+ and V
D
+ supply lines can cause
conversion errors; the comparator will respond to the noise.
The ADC is especially sensitive to any power supply spikes
that occur during the auto-zero or linearity correction. The
minimum power supply bypassing capacitors recommended
are low inductance tantalum capacitors of 10 µF or greater
paralleled with 0.1 µF monolithic ceramic capacitors. More or
different bypassing may be necessary depending on the
overall system requirements. Separate bypass capacitors
should be used for the V
A
+ and V
D
+ supplies and placed as
close as possible to these pins.
01135445
FIGURE 17. V
REF Operating Range
www.national.com
34


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