APPLICATION BULLETIN ® 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 PROGRAMMABLE-GAIN INSTRUMENTATION AMPLIFIERS By David Jones (602) 746-7696 and R. Mark Stitt The INA115 is a precision instrumentation amplifier (IA). Its gain-sense and amplifier output connections are available so that gains can be accurately set using external resistors. By adding switches or a multiplexer, you can make a precision programmable-gain IA (PGIA). Using the circuit topology shown in this bulletin, the switches are in series with op amp inputs so their resistance does not add error. If you need an IA with more gain steps, you can cascade two or more PGIAs. For example, if you cascade two PGA205s you will get gains of 1, 2, 4, 8, 16, 32, and 64V/V. For other gains or gain-steps, you may want to make your own PGIA using an INA115. The circuits and equations below make it easy to design a programmable-gain IA with any gains and any number of gain steps. Burr-Brown offers fixed and programmable-gain IAs with amplifiers similar to the INA115. If you need a fixed gain of 100V/V, use the INA131. If you need a digitally programmable IA with decade gains of 1, 10, 100, and 1000V/V, or binary gains of 1, 2, 4, and 8V/V, use the PGA204 or PGA205. MODEL GAINS (V/V) INA131 PGA204 PGA205 100 1, 10, 100, 1000 1, 2, 4, 8 TABLE I. Fixed and Programmable-Gain IAs. R0 1 VO1 R1 VIN– G1 R2 Rm 2 G2 INA115 4 Amp B Gm Gn NC 3 12 Amp C Rn NC Rm R2 14 11 VOUT Ref 10 Gn Gm 15 G2 G1 VIN+ 5 Amp A R1 8 VO2 R0 FIGURE 1. Programmable-Gain Instrumentation Amplifier with n Gain Steps and Lowest Gain > 1V/V. © 1992 Burr-Brown Corporation AB-042 Printed in U.S.A. September, 1992 If you want a PGIA with n gains and the first gain is not unity (1V/V), use the following relationships and the circuit shown in Figure 1. If you want to use the 25kΩ feedback resistors in the INA115 for the R0s, you can use the circuit shown in Figure 2. Keep in mind that the gain accuracy and gain drift will be limited by the internal feedback resistors. The 25kΩ feedback resistors have a tolerance of ±0.5% with a temperature coefficient of resistance drift (TCR) of up to 100ppm/°C. In the INA131, PGA204 and PGA205, resistor matching and TCR tracking of the resistors on the die give typical gain error and drift of 0.01% and 5ppm/°C. RESISTOR VALUES FOR PGIA WITH n GAINS AND LOWEST GAIN > 1V/V R0 = Your choice (e.g. 25kΩ) R0 = 25kΩ when using the internal feedback resistors in the INA115 (see Figure 2) R0 (G1 – G2) R1 = G (1 – G ) 2 1 G1 R0 (G2 – G3) R2 = G G (1 – G ) 2 3 1 G1 R0 (Gm – Gm+1) Rm = G G (1 – G ) m m+1 1 GAINS (V/V) R0 (Ω) R1 (Ω) R2 (Ω) R3 (Ω) R4 (Ω) 2, 4, 8, 16 10, 20, 50, 100 3dB, 6dB, 9dB, 12db 10, 100, 1k, 10k 10, 100, 200, 500 100, 200, 400, 800 25k 25k 25k 100k 49.9k 100k 12.4k 1.4k 17.8k 10k 4.99k 511 6.19k 825 12.4k 1k 280 255 3.09k 280 8.87k 100 165 127 6.19k 562 43.2k 22.1 221 255 NOTE: Nearest Standard 1% Resistor Values. 2 G1 R0 Rn = G (G – 1) n 1 TABLE II. Examples of Resistor Values for Selected Gains—Figure 1 and 2 Circuits. Where: Gm = Intermediate gain (V/V) Gn = Highest gain (V/V) R0, R1, ... Rn = Resistor value per circuit diagram (Ω) NC 1 VO1 R1 VIN– G1 R2 2 G2 INA115 4 Amp B Gm Rm Gn 3 12 R0 Amp C Rn 14 R0 11 VOUT Ref 10 Rm R2 Gn Gm 15 G2 G1 VIN+ 5 Amp A R1 8 VO2 NC FIGURE 2. Programmable-Gain Instrumentation Amplifier with n Gain Steps and Lowest Gain > 1V/V. This circuit uses the 25kΩ feedback resistor in the INA115 for R0 of Figure 1. 2 If you want a PGIA with n gains and the first gain is unity (1V/V), use the following relationships and the circuit shown in Figure 3. If you want to use the 25kΩ feedback resistors in the INA115 for the R1s, you can use the circuit shown in Figure 4. Keep in mind the gain accuracy and gain drift limitations discussed previously. RESISTOR VALUES FOR PGIA WITH n GAINS AND LOWEST GAIN = 1V/V R0 = 0 An actual example of a four-gain digitally programmable IA is shown in Figure 5. It uses a four-channel differential multiplexer (MUX) for gain switching. R1 = Your choice (e.g. 25kΩ) R1 = 25kΩ when using the internal feedback resistors in the INA115 (see Figure 4) R2 = R1 (G2 – G3) G3 (1 – G2) R3 = G2 R1 (G3 – G4) G3 G4(1 – G2) GAINS (V/V) R0 (Ω) R1 (Ω) 1, 2, 4, 8 1, 8, 64, 512 1, 2, 5, 10 1, 10, 100, 1k 1, 10, 100, 200 0dB, 3dB, 6dB, 9dB 0 0 0 0 0 0 25k 25k 25k 49.9k 20k 24.9k R2 (Ω) R3 (Ω) 12.4k 6.19k 3.09 392 15k 4.99k 4.99k 499 2k 110 17.8k 12.4k R4 (Ω) 12.4k 113 10k 110 221 60.4k NOTE: Nearest Standard 1% Resistor Values. Rm = G2 R1 (Gm – Gm+1) Gm Gm+1(1 – G2) Rn = 2 G2 R1 Gn (G2 – 1) TABLE III. Examples of Resistor Values for Selected Gains— Figure 3 and 4 Circuits. Where: Gm = Intermediate gain (V/V) Gn = Highest gain (V/V) R0, R1, ... Rn = Resistor value per circuit diagram (Ω) 1 VO1 R1 VIN– G1 R2 Rm 2 G2 INA115 4 Amp B Gm Gn NC 3 12 Amp C Rn NC Rm R2 11 Ref 14 10 Gn Gm 15 G2 G1 VIN+ 5 Amp A R1 8 VO2 FIGURE 3. Programmable-Gain Instrumentation Amplifier with n Gain Steps and Lowest Gain = 1V/V. 3 VOUT 1 VO1 VIN– G1 R2 INA115 4 Amp B 2 G2 Gm Rm Gn 3 12 R1 11 Amp C Rn R1 14 VOUT Ref 10 Rm R2 Gn Gm 15 G2 G1 VIN+ Amp A 5 8 VO2 FIGURE 4. Programmable-Gain Instrumentation Amplifier with n Gain Steps and Lowest Gain = 1V/V. This circuit uses the 25kΩ feedback resistor in the INA115 for R1 of Figure 3. R1 49.9kΩ R2 4.99kΩ 4 5 R3 499Ω 6 7 1 R4 110Ω 16 10 R3 499Ω 11 12 13 R2 4.99kΩ HI-509 In1A In2A In3A +V En OutA 14 2 VCC VDD 8 In– 2 In4A A0 3 Gnd 1 GS1 VO1 RG1 FB 12 VO INA115 14 15 In4B OutB – VIN 15 A1 In3B 4 9 In+ 5 RG2 GS2 + VIN VO2 11 VO Ref 10 8 In2B In1B –V 3 VEE R1 49.9kΩ A1 A0 GAIN L L H H L H L H 1 10 100 1000 FIGURE 5. Programmable-Gain Instrumentation Amplifier Example with Four Gain Steps and Lowest Gain = 1V/V. The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes no responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject to change without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant any BURR-BROWN product for use in life support devices and/or systems. 4

- Similar pages
- ETC AB-001
- ETC AB-091
- ETC AB-010
- ETC AB-002
- ETC AB-096
- ETC DEM-OPA268XU
- BB INA104AM
- ETC AB-076
- ETC AB-137
- ETC AB-041
- BB PGA204/PGA205
- BB PGA103U
- ETC AB-056
- BB INA115AU
- BB ISO508P
- ETC AB-024
- ETC AB-151
- ISL28533, ISL28534, ISL28535, ISL28633, ISL28634
- ETC AB-018
- ETC AB-058
- ETC AB-016
- ETC AB-088