廬
APPLICATION BULLETIN
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
INPUT FILTERING THE INA117
鹵
200V DIFFERENCE AMPLIFIER
Many customers have asked how to add input filtering to the
INA117. Since the INA117 is rated for
鹵200V
input voltage
(鹵500V without damage), it is commonly used in environ-
ments with very high input noise or with high-voltage input
transients. This bulletin shows how to connect input filters,
discusses the errors they can add, and shows how to elimi-
nate the errors.
Figure 1 shows the connection of a differential input filter.
A pole is formed by C
1
and the two external input resistors.
f
鈥?dB
= 1/(4 鈥?/div>
蟺
鈥?R
I
鈥?C
1
). Differential input filtering is
preferred because mismatches in filter components do not
degrade CMR.
DON鈥橳 USE COMMON-MODE
INPUT FILTERS ALONE
Don鈥檛 be tempted to use common-mode input filtering alone
(Figure 2) unless you are prepared to carefully match com-
ponents. Mismatches between the R
I
鈥?C
2
time constants
reduce AC CMR. The mismatches result in a differential
input signal in response to AC common-mode inputs. Even
if you successfully match the components for good AC
CMR at room temperature, maintaining the match over
temperature can be a problem.
A COMBINATION COMMON-MODE
AND DIFFERENTIAL INPUT FILTER IS OK
If you want common-mode input filtering, use it in conjunc-
tion with differential input filtering as shown in Figure 3. If
R
I
2k鈩?/div>
V
2
R
I
2k鈩?/div>
V
3
C
2
0.01碌F
3
2
R
1
380k鈩?/div>
R
2
380k鈩?/div>
R
3
380k鈩?/div>
6
V
O
R
4
20.0k鈩?/div>
INA117
5
R
5
21.1k鈩?/div>
8
1
Don鈥檛 use this common-mode input filter alone. AC CMR will be de-
graded by mismatches in the filter time constants.
FIGURE 2. INA117 with Common-Mode Input Filter.
R
I
2k鈩?/div>
V
2
R
I
2k鈩?/div>
V
3
C
2
0.01碌F
C
1
4.7碌F
2
R
1
380k鈩?/div>
R
2
380k鈩?/div>
R
3
380k鈩?/div>
3
R
5
21.1k鈩?/div>
8
1
R
4
20.0k鈩?/div>
INA117
5
6
V
O
R
I
2k鈩?/div>
V
2
R
I
2k鈩?/div>
V
3
3
2
C
1
R
1
380k鈩?/div>
R
2
380k鈩?/div>
If C
1
>> C
2
, AC common-mode errors due to R
I
鈥?C
2
mismatches will be
shunted by C
1
, improving AC CMR.
R
3
380k鈩?/div>
6
V
O
R
4
20.0k鈩?/div>
INA117
5
FIGURE 3. INA117 with a Combination Differential and
Common-Mode Input Filter.
C
1
>> C
2
, AC common-mode errors will be shunted by C
1
so AC CMR can be successfully boosted. A value of
C
1
= 500 鈥?C
2
is suggested.
Figure 4 shows actual CMR vs Frequency performance plots
for the Figure 2 and Figure 3 circuits. Standard INA117
performance is shown for comparison. The standard INA117
has about 60dB CMR at 30kHz. Mismatches of 5% in R 鈥?C
time constants (5% C
2
mismatch) cause the Figure 2 circuit
CMR to drop below 60dB at less than 200Hz. Adding a
AB-029
Printed in U.S.A. March, 1991
R
5
21.1k鈩?/div>
8
1
A differential pole is formed at:
f
鈥?dB
= 1/(4 鈥?/div>
蟺
鈥?R
I
鈥?C
1
)
AC CMR is not degraded by a differential input filter.
FIGURE 1. INA117 with Differential Input Filter.
漏
1991 Burr-Brown Corporation
AB-029相關(guān)型號PDF文件下載
-
型號
版本
描述
廠商
下載
-
英文版
AB-001 - INCREASING INA117 DIFFERENTIAL INPUT RANGE
ETC
-
英文版
AB-002 - MAKE A PRECISION CURRENT SOURCE OR CURRENT SINK
ETC
-
英文版
AB-003 - VOLTAGE-REFERENCE FILTERS
ETC
-
英文版
AB-004 - MAKE A PRECISION -10V REFERENCE
ETC
-
英文版
AB-005 - Make A Precision -10V Reference
ETC
-
英文版
AB-006 - Make a -10V to +10V Adjustable Precision Voltage So...
ETC
-
英文版
AB-007 - CLASSICAL OP AMP OR CURRENT-FEEDBACK OP AMP?
ETC
-
英文版
AB-008 - AC COUPLING INSTRUMENTATION AND DIFFERENCE AMPLIFIE...
ETC
-
英文版
AB-009 - SINGLE-SUPPLY OPERATION OF ISOLATION AMPLIFIERS
ETC
-
英文版
AB-010 - -200V DIFFERENCE AMPLIFIER WITH COMMON-MODE VOLTAGE...
ETC
-
英文版
AB-011 - LOW POWER SUPPLY VOLTAGE OPERATION OF REF102 10.0V ...
ETC
-
英文版
AB-012 - BOOST ISO120 BANDWIDTH TO MORE THAN 100kHz
ETC
-
英文版
AB-013 - INCREASING ADC603 INPUT RANGE
ETC
-
英文版
AB-014 - INPUT OVERLOAD PROTECTION FOR THE RCV420 4-20mA CUR...
ETC
-
英文版
AB-015 - EXTENDING THE COMMON-MODE RANGE OF DIFFERENCE AMPLI...
ETC
-
英文版
AB-016 - BOOST AMPLIFIER OUTPUT SWING WITH SIMPLE MODIFICATI...
ETC
-
英文版
AB-017 - Sallen-Key Low-Pass Filter Design Program
ETC
-
英文版
AB-018 - 0 TO 20mA RECEIVER USING RCV420
ETC
-
英文版
AB-019 - USING THE ADS7800 12 BIT ADC WITH UNIPOLAR INPUT SI...
ETC
-
英文版
AB-020 - BURR-BROWN SPICE BASED MACROMODELS. REV. F
ETC
1
2
3
4
