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| 型號： | AB-019 |
| 英文描述： | AB-019 - USING THE ADS7800 12 BIT ADC WITH UNIPOLAR INPUT SIGNALS |
| 中文描述： | 抗體- 019 -使用ADS7800與單極性輸入信號12位ADC |
| 文件頁數(shù)： | 1/5頁 |
| 文件大?。?/td> | 57K |
| 代理商： | AB-019 |
EXTENDING THE COMMON-MODE RANGE
OF DIFFERENCE AMPLIFIERS
By R. Mark Stitt (602) 746-7445
APPLICATION BULLE TIN
Mailing Address: PO Box 11400 Tucson, AZ 85734 Street Address: 6730 S. Tucson Blvd. Tucson, AZ 85706
Tel: (602) 746-1111 Twx: 910-952-111 Telex: 066-6491 FAX (602) 889-1510 Immediate Product Info: (800) 548-6132
Extending the common-mode range of difference amplifiers
allows their use in a wider variety of reduced power-supply
applications.
The INA117 has a specified common-mode input range of
±
200V when operating on standard
±
15V power supplies. At
power-supply voltages above
±
13V, the INA117 input range
is limited to
±
200V by the power capabilities of its internal
input resistors. On reduced power supplies, the input range
is limited by the common mode input range of the internal
op amp.
The linear common-mode input range of the internal op amp
extends to within 3V of its power supply voltage. For
example, with a
±
15V power supply, the common-mode
input range of the internal op amp is
±
12V. Because the
INA117 internal resistor network divides the input by 20, the
actual input range of the INA117 would be 20 (
±
12V), or
240V, for
±
15V power supplies. Similarly, reducing the
power supply voltage to
±
6V will limit the input common-
mode voltage to
±
60V.
There are two approaches to boosting the common-mode
input range for reduced power supply applications: Offset-
ting the common-mode range by a fixed amount, and dy-
namically adjusting the common-mode range to follow the
input common-mode signal.
OFFSETTING THE INPUT COMMON-MODE
RANGE WITH A CONSTANT VOLTAGE
In many applications, the common-mode signal range is
known and the common-mode input range of the difference
amplifier can be adjusted to coincide with the required
range. For example, the
±
60V common-mode range of the
INA117 operating on
±
6V supplies could be shifted to range
from +0V to +120V, or +50V to +170V.
To offset the common-mode range, the reference connection
of the difference amplifier is connected to an offsetting
voltage, V
, instead of ground. With the reference connected
to an offsetting potential, a second difference amplifier must
be used to refer the output back to ground.
One way to offset the input voltage is to connect the
reference pins 1 and 5 to the negative supply voltage as
shown in Figure 1. Another possibility is to derive the offset
voltage from a zener diode connected to the negative power
supply as shown in Figure 2. In either case, the total
common-mode range of the INA117 is unchanged and
shifted by –19 (V
X
).
380k
380k
380k
21.1k
20k
2
3
8
1
5
6
V
3
V
2
INA117
4
INA105
25k
25k
25k
25k
4
2
7
7
5
6
1
V = V – V
–3V > V
O
> –10V
Swap A
2
pins 2 and 3 for:
10V > V
O
> 3V
also;
((V+) – 5V) > V
O
> ((V–) + 3V)
V+
V–
3
R
1
R
2
R
4
R
5
R
3
V = V–
R
1
R
2
R
3
R
4
A
1
A
2
FIGURE 1. Offsetting the INA117 Common-Mode Input Range Using the Negative Power Supply as a Reference.
AB-015
1990 Burr-Brown Corporation
Printed in U.S.A. August, 1990
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