Datasheet MAT02 (Analog Devices) - 10

MAT02
Substituting in the voltage relationships and simplifying leads to: m R   O V Z V V O = , where R Y V   1 X (20) R + B KRA m = R + 1– K ( )R B A The factor “K” is a potentiometer position and varies from zero to 1.0, so “m” ranges from RB/(RA + RB) to (RB + RA)/RB. Practical values are 125 Ω for RB and 500 Ω for RA; these values will provide an adjustment range of 0.2 to 5.0. A value of 100 kΩ is recommended for the R1 resistors assuming a full- scale input range of 10 V. As with the one-quadrant multiplier/divider circuit previously discussed, the VX, VY, and VZ inputs must all be positive. The op amps should have the lowest possible input offsets. The Figure 7. Fast Logarithmic Amplifier OP07 is recommended for most applications, although such programmable micropower op amps as the OP193/OP293 offer
LOW-NOISE
ⴛ
1000 AMPLIFIER
advantages in low-power or single-supply circuits. The micro- The MAT02 noise voltage is exceptionally low, only 1 nV/√Hz power op amps also have very low input bias-current drift, an at 10 Hz when operated over a collector current range of 1 mA important advantage in log/antilog circuits. External offset to 4 mA. A single-ended ×1000 amplifier that takes advantage of nulling may be needed, particularly for applications requiring a this low MAT02 noise level is shown in Figure 8. In addition to wide dynamic range. Frequency compensating capacitors, on low noise, the amplifier has very low drift and high CMRR. An the order of 50 pF, may be required for A2 and A3. Amplifier OP184 is used for the second stage to obtain good speed with A1 is likely to need a larger capacitor, typically 0.0047 µF, to minimal power consumption. Small-signal bandwidth is 4.0 assure stability. MHz, slew rate is 2.4 V/µs, and total supply current is approxi- Accuracy is limited at the higher input levels by bulk emitter mately 2.25 mA. resistance, but this is much lower for the MAT02 than for other transistor pairs. Accuracy at the lower signal levels primarily depends on the op amp offsets. Accuracies of better than 1% are readily achievable with this circuit configuration and can be better than ± 0.1% over a limited operating range.
FAST LOGARITHMIC AMPLIFIER
The circuit of Figure 7 is a modification of a standard logarith- mic amplifier configuration. Running the MAT02 at 2.5 mA per side (full-scale) allows a fast response with wide dynamic range. The circuit has a 7 decade current range, a 5 decade voltage range, and is capable of 2.5 µs settling time to 1% with a 1 V to 10 V step. The output follows the equation:
OBSOLETE
R + 3 R2 kT V REF V In O = (21) R2 q V IN The output is inverted with respect to the input, and is nomi- nally –1 V/decade using the component values indicated. Figure 8. Low-Noise, Single-Ended × 1000 Amplifier –10– REV. E