7

®

XTR104

BRIDGE BALANCE

ment can be used to compensate for the initial accuracy of

the bridge and/or to trim the offset voltage of the XTR104.

bridge, and the trim range required. This trim circuit places

can be calculated:

(3)

Make R

Figure 2 shows another way to adjust zero errors using the

output current adjustment pins of the XTR104. This pro-

vides

±500µA (typical) adjustment around the initial low-

scale output current. This is an output current adjustment

input stage is set for high gain the output current adjustment

may not provide sufficient range.

bridge excitation voltage can be made to vary as much as

±0.5V in response to the bridge output voltage. Be sure that

the total load on the V

Signal-dependent variation of the bridge excitation voltage

provides a second-order term to the complete transfer func-

tion (including the bridge). This can be tailored to correct for

bridge sensor nonlinearity. Either polarity of nonlinearity

(bowing up or down) can be compensated by proper connec-

which compensates for a positive bow in the bridge re-

decrease with increasing bridge output, to compensate for

negative-bowing nonlinearity.

To determine the required value for R

nonlinearity of the bridge sensor with constant excitation

voltage. The linearization circuitry can only compensate for

the parabolic portion of a sensor’s nonlinearity. Parabolic

nonlinearity has a maximum deviation from linear occurring

at mid-scale (see Figure 4). Sensors with nonlinearity curves

similar to that shown in Figure 4, but not peaking exactly at

mid-scale can be substantially improved. A nonlinearity that

is perfectly “S-shaped” (equal positive and negative

nonlinearity) cannot be corrected with the XTR104. It may,

however, be possible to improve the worst-case nonlinearity

of a sensor by equalizing the positive and negative

nonlinearity.

The nonlinearity, B (in % of full scale), is positive or

negative depending on the direction of the bow. A maximum

of

±2.5% nonlinearity can be corrected. An approximate

(5)

V

constant 5V excitation.

B is the parabolic nonlinearity in

±% of full scale.

Methods for refining this calculation involve determining

later).

B is a signed number (negative for a downward-bowing

this case, use the resistor value indicated (ignore the sign),

Figure 3.

about a 5:1 improvement in bridge nonlinearity.

Example: The bridge sensor depicted by the negative-

bowing curve in Figure 4. Its full scale output is 10mV with

constant 5V excitation. Its maximum nonlinearity, B, is

–1.9% referred to full scale (occurring at mid-scale). Using

equation 5:

R

1

≈

5V • R

B

4• V

TRIM

15

14

XTR104

16

10k

Ω

XTR104

5k

Ω

5k

Ω

16

(a)

(b)

15

14

±500µA typical

output current

adjustment range.

±50µA typical

output current

adjustment range.

FIGURE 2. Low-scale Output Current Adjustment.

R

LIN

=

K

LIN

•V

FS

0. 2 • B

LINEARIZATION

LIN

vary according to the following equation:

(4)

differential inputs (in V).

variations in the fabrication of the XTR104).

V

R

= 5V + V

LIN

K

LIN

R

LIN