Analog Devices AN532 Application Notes

AN-532

a

APPLICATION NOTE

One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106 • 781/329-4700 • World Wide Web Site: http://www.analog.com

Applying Model 5B38 with 120 V Strain Gages

INTRODUCTION

The Model 5B38 isolated 10 V excitation voltage is rated
to drive strain gage and load cell impedances as low as
300 Ω. However, the 5B38 excitation voltage can easily
be applied to drive a 120 Ω bridge with the simple addi­tion of two resistors in series with the excitation supply,
as shown in Figure 1. This approach is based on the un­derstanding that the 5B38 excitation voltage is a regu­lated supply voltage within its rated load from 10 kΩ to
300 Ω. It also recognizes that 120 Ω strain gages typically
operate with an excitation less than 10 V to reduce ex­cessive internal power dissipation that would cause
bridge self-heating effects with resulting thermal insta­bility during operation.

Procedure To Calculate Series Resistance:

1. Using the maximum load resistance rating of the
5B38 excitation voltage (300 Ω), and the load resis­tance of the strain gage (120 Ω), the total added series
resistance is calculated as follows:

R

MAX LOAD

–

R

=

BRIDGE

300 Ω –120 Ω = 180 Ω

R

SERIES TOTAL

Strain Gage Excitation Voltage

The resulting bridge excitation voltage (e.g., the excita­tion voltage seen directly across the 120 Ω bridge), is
reduced from the 5B38 10 V to: [120 Ω/300 Ω] × [10 V] =

4.0 V. Applications with low impedance bridges below
350 Ω, such as the 120 Ω strain gage in this example,
typically work well with an excitation voltage of less
than 10 volts. Note the power dissipation of a 120 Ω
bridge will decrease by a factor of six—from 830 mW to
130 mW—when the excitation voltage is reduced from
10 V to 4.0 V. The benefit is reduced thermal, or self­heating effects of the strain gage itself. However, it is
always best to contact the strain gage manufacturer and
verify that the 4.0 excitation voltage is acceptable for a
specific strain gage or load cell model.

+EXC

HI

5B38

LO

–EXC

90V

90V

R

R

B

B

R

R

B

B

E3282–0–3/98

2. The calculated total series resistance is then divided
in two equal value resistors—[180 Ω/2 = 90Ω]—each
of which is placed in series with the 5B38 excitation
supply terminals, +EXC and –EXC, and the 120 Ω
strain gage.

• The calculated 180 Ω series additional resistance is

split into two equal parts, to provide the strain gage
with an excitation voltage that is balanced between
the common or midpoint of the bridge voltage. This
will eliminate common-mode voltage (CMV) error
by establishing the bridge input CMV at 0 V.

With this arrangement, the maximum load resistance
rating of the 5B38 excitation voltage is maintained: e.g.,
90 Ω + 90 Ω + 120 Ω = 300 Ω.

Figure 1. Resist or Connections to 5B38 and 120 Ω Bridge

PRINTED IN U.S.A.