ADC12062EVAL National Semiconductor, ADC12062EVAL Datasheet - Page 12

• Manufacturer Part Number: ADC12062EVAL
• Description: BOARD EVALUATION FOR ADC12062
• Manufacturer: National Semiconductor

Specifications of ADC12062EVAL

• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: *ADC12062EVAL

www.national.com
Applications Information
2.0 THE ANALOG INPUT
The analog input of the ADC12062 can be modeled as two
small resistances in series with the capacitance of the input
hold capacitor (C
closed during the Sample period, and open during Hold. The
source has to charge C
sample period. Note that the source impedance of the input
voltage (R
charge C
will not settle to within 0.5 LSBs of V
conversion begins, and the conversion results will be incor-
rect. From a dynamic performance viewpoint, the combina-
tion of R
Minimizing R
the input stage of the converter.
SOURCE
IN
SOURCE
. If R
SOURCE
SOURCE
, R
IN
) has a direct effect on the time it takes to
), as shown in Figure 7 . The S/H switch is
MUX
will increase the frequency response of
, R
is too large, the voltage across C
IN
SW
to the input voltage within the
, and C
IN
form a low pass filter.
SOURCE
FIGURE 5. The Capacitive Voltage Divider
(Continued)
FIGURE 6. ADC Control Logic
before the
IN
12
Typical values for the components shown in Figure 7 are:
R
time to n bits is:
The bandwidth of the input circuit is:
For maximum performance, the impedance of the source
driving the ADC12062 should be made as small as possible.
A source impedance of 100 or less is recommended. A plot
of dynamic performance vs. source impedance is given in
the Typical Performance Characteristics section.
If the signal source has a high output impedance, its output
should be buffered with an operational amplifier capable of
driving a switched 25 pF/100
ties at the op amp’s output during the sampling period can
result in conversion errors. The LM6361 high speed op amp
MUX
t
f
SETTLE
−3dB
= 100 , R
= 1/(2
= (R
SOURCE
*
SW
3.14
= 100 , and C
*
+ R
(R
SOURCE
MUX
01149016
load. Any ringing or instabili-
+ R
+ R
01149015
SW
IN
)
MUX
= 25 pF. The settling
*
C
+ R
IN
*
SW
n
)
*
*
ln (2).
C
IN
)