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XTR104AP Datasheet(PDF) 7 Page - Burr-Brown (TI) |
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XTR104AP Datasheet(HTML) 7 Page - Burr-Brown (TI) |
7 / 11 page 7 ® XTR104 BRIDGE BALANCE Figure 1 shows a bridge trim circuit (R1, R2). This adjust- ment can be used to compensate for the initial accuracy of the bridge and/or to trim the offset voltage of the XTR104. The values of R1 and R2 depend on the impedance of the bridge, and the trim range required. This trim circuit places an additional load on the VR output. The effective load of the trim circuit is nearly equal to R2. Total load on the VR output terminal must not exceed 2mA. An approximate value for R1 can be calculated: (3) Where: RB is the resistance of the bridge. VTRIM is the desired ±voltage trim range (in V). Make R 2 equal or lower in value to R1. 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 that is independent of the input stage gain set with RG. If the input stage is set for high gain the output current adjustment may not provide sufficient range. With V+LIN and V–LIN connected to the bridge output, the 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 R output is less than 2mA at the maximum excitation voltage, VR = 5.5V. 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- tion of the VLIN inputs. Connecting V+LIN to V+IN and V–LIN to V–IN (Figure 1) causes VR to increase with bridge output which compensates for a positive bow in the bridge re- sponse. Reversing the connections (Figure 3) causes VR to decrease with increasing bridge output, to compensate for negative-bowing nonlinearity. To determine the required value for R LIN you must know the 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 value for RLIN can be calculated by: (5) Where: KLIN ≈ 24000. V FS is the full-scale bridge output (in Volts) with constant 5V excitation. B is the parabolic nonlinearity in ±% of full scale. RLIN in Ω. Methods for refining this calculation involve determining the actual value of KLIN for a particular device (explained later). B is a signed number (negative for a downward-bowing nonlinearity). This can produce a negative value for RLIN. In this case, use the resistor value indicated (ignore the sign), but connect V+LIN to V–IN and V–LIN to V+IN as shown in Figure 3. This approximate calculation of RLIN generally provides 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 Differential voltage applied to the linearization inputs, V+ LIN and V–LIN, causes the reference (excitation) voltage, VR, to vary according to the following equation: (4) Where: VLIN is the voltage applied to the V+LIN and V–LIN differential inputs (in V). RLIN in Ω. KLIN ≈ 24000 (approximately ±20% depending on variations in the fabrication of the XTR104). V R = 5V + V LIN K LIN R LIN |
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