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OP2177ARM Datasheet(PDF) 15 Page - Analog Devices |
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OP2177ARM Datasheet(HTML) 15 Page - Analog Devices |
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15 / 20 page  REV. B OP1177/OP2177/OP4177 –15– 200 +15V 15V 7 4 6 5 8 1/2 OP2177 VOUT 500 4.37k 100 100 20 4.12k 4.12k 5k 49.9k ADR421 +15V 0.1 F +15V 15V 1 4 2 3 8 VOUT 100 RTD 1/2 OP2177 Figure 15. Low Power Linearized RTD Circuit Single Op Amp Bridge The low input offset voltage drift of the OP1177 makes it very effective for bridge amplifier circuits used in RTD signal condi- tioning. It is often more economical to use a single bridge op amp as opposed to an instrumentation amplifier. In the circuit of Figure 16, the output voltage at the op amp is: V R R V R R R R O REF = ++ + () 2 1 1 1 2 1 δ δ where δ = ∆R/R is the fractional deviation of the RTD resis- tance with respect to the bridge resistance due to the change in temperature at the RTD. For δ << 1, the expression above becomes: V R R V R R R R R R R R R R V O REF REF ≅ ++ = + + 2 1 11 2 2 1 1 2 1 2 δ δ With VREF constant, the output voltage is linearly proportional to δ with a gain factor of: V R R R R R R REF 2 1 1 2 1 2 + + VOUT 1 4 2 3 OP1177 7 V+ V RF RF R R R R(1+ ) ADR421 15V 0.1 F Figure 16. Single Bridge Amplifier REALIZATION OF ACTIVE FILTERS Bandpass KRC or Sallen-Key Filter The low offset voltage and the high CMRR of the OP1177 make it an excellent choice for precision filters such as the KRC filter shown in Figure 17. This filter type offers the capability to tune the gain and the cutoff frequency independently. Since the common-mode voltage into the amplifier varies with the input signal in the KRC filter circuit, a high CMRR is required to minimize distortion. Also, the low offset voltage of the OP1177 allows a wider dynamic range when the circuit gain is chosen to be high. The circuit of Figure 17 consists of two stages. The first stage is a simple high-pass filter whose corner frequency fC is: 1 2 1 212 π CC R R (2) and whose QK R R = 1 2 (3) where K is the dc gain. Choosing equal capacitor values minimizes the sensitivity and simplifies Equation 2 to: 1 21 2 πCR R The value of Q determines the peaking of the gain versus frequency (ringing in transient response). Commonly chosen values for Q are generally near unity. Setting Q = 1 2 , yields minimum gain peaking and minimum ringing. Determine values for R1 and R2 by use of Equation 3. For Q = 1 2 , R1/R2 = 2 in the circuit example. Pick R1 = 5 k Ω and R2 = 10 k Ω for simplicity. The second stage is a low-pass filter whose corner frequency can be determined in a similar fashion. For R3 = R4 = R. f C R C C Q C C == 1 2 3 4 1 2 3 4 π and Channel Separation Multiple amplifiers on a single die are often required to reject any signals originating from the inputs or outputs of adjacent channels. OP2177 input and bias circuitry is designed to prevent feedthrough of signals from one amplifier channel to the other. As a result the OP2177 has an impressive channel separation of greater than –120 dB for frequencies up to 100 kHz and greater than –115 dB for signals up to 1 MHz. |
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