廬
APPLICATION BULLETIN
INCREASING ADC603 INPUT RANGE
By R. Mark Stitt, (602) 746-7445
Mailing Address: PO Box 11400 鈥?Tucson, AZ 85734 鈥?Street Address: 6730 S. Tucson Blvd. 鈥?Tucson, AZ 85706
Tel: (602) 746-1111 鈥?Twx: 910-952-111 鈥?Telex: 066-6491 鈥?FAX (602) 889-1510 鈥?Immediate Product Info: (800) 548-6132
The ADC603 is a 10MHz, 12-bit analog-to-digital converter
with a
鹵1.25V
input range. Many applications call for a
higher input range such as
鹵2.5V.
A resistor divider can be
used as an input attenuator to increase the input range. The
OPA620 can be used to buffer the input attenuator for high-
source-impedance applications. Suggested component val-
ues and measured performance results are shown in this
bulletin.
Since the ADC603 has a high-impedance input, a simple
voltage divider as shown in Figure 1 can be used to increase
its voltage input range. The source impedance of the divider
as seen by the ADC603 is R
1
|| R
2
(the parallel combination
of R
1
and R
2
). A divider source impedance of 50鈩?is
recommended since it has been shown to give consistently
good results. If a higher divider input impedance is needed
and adding a buffer is not viable, source impedances up to
500鈩?should give satisfactory results. If hardware gain trim
is needed, select the next higher 1% resistor value for R
1
and
use a 10k鈩?multi-turn trim pot in parallel with R
1
for gain
trim.
V
IN
R
2
100鈩?/div>
45
R
1
100鈩?/div>
46
ADC603
12 Bits
Out
V
IN
R
2
100鈩?/div>
R
3
66.5鈩?/div>
R
1
100鈩?/div>
45
46
ADC603
12 Bits
Out
FIGURE 2. ADC603 12-Bit ADC with Three-Resistor 2/1
Input Attenuator to Provide
鹵2.5V
Input Range
and 50鈩?Termination Impedance.
2
3
+5V
7
OPA620
4
6
V
IN
(鹵2.5V)
R
2
100鈩?/div>
45
R
1
100鈩?/div>
46
3
ADC603
6
12 Bits
Out
鈥?.2V
FIGURE 1. ADC603 12-Bit ADC with 2/1 Input Attenuator
to Provide
鹵2.5V
Input Range.
If an input impedance of 50鈩?to the circuit is needed as a
termination, add a third resistor as shown in Figure 2. The
three-resistor approach improves accuracy by placing the
majority of the termination power dissipation in the third
resistor. This minimizes error-producing self heating in the
precision divider network. Pay attention to the power rating
for R
3
. For a
鹵10V
input, R
3
must be rated 2W.
If a high input impedance is needed, drive the divider with
a unity-gain-connected OPA620 buffer amp as shown in
Figure 3. The OPA620 can be used for inputs as high as
鹵3V.
FIGURE 3. ADC603 12-Bit ADC with 2/1 Input Attenuator
to Provide High Input Impedance
鹵2.5V
Input
Range.
Equations for determining recommended resistor values are:
R
1
= 50鈩?鈥?N/(N 鈥?1)
R
2
= (N 鈥?1) 鈥?R
1
R
3
= 50鈩?鈥?(R
1
+ R
2
)/(R
1
+ R
2
鈥?50鈩?
Where:
R
1
, R
2
, R
3
are in
鈩?/div>
N = input divider ratio
漏
1990 Burr-Brown Corporation
AB-013
Printed in U.S.A. July, 1990
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