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AD549JH Datasheet(PDF) 11 Page - Analog Devices |
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AD549JH Datasheet(HTML) 11 Page - Analog Devices |
11 / 12 page AD549 REV. A –11– Figure 43. Photodiode Preamp Noise Sources Figure 44. Photodiode Preamp Noise Sources’ Spectral Density vs. Frequency Log Ratio Amplifier Logarithmic ratio circuits are useful for processing signals with wide dynamic range. The AD549L’s 60 fA maximum input cur- rent makes it possible to build a log ratio amplifier with 1% log conformance for input current ranging from 10 pA to 1 mA, a dynamic range of 160 dB. The log ratio amplifier in Figure 45 provides an output voltage proportional to the log base 10 of the ratio of the input currents I1 and I2. Resistors R1 and R2 are provided for voltage inputs. Since NPN devices are used in the feedback loop of the front- end amplifiers that provide the log transfer function, the output is valid only for positive input voltages and input currents. The input currents set the collector currents IC1 and IC2 of a matched pair of log transistors Q1 and Q2 to develop voltages VA and VB: VA, B = – (kT/q) ln IC/IES where IES is the transistors’ saturation current. The difference of VA and VB is taken by the subtractor section to obtain: VC = (kT/q) ln (IC2/IC1) VC is scaled up by the ratio of (R9 + R10)/R8, which is equal to approximately 16 at room temperature, resulting in the output voltage: VOUT = 1 × log (IC2/IC1) V. R8 is a resistor with a positive 3500 ppm/ °C temperature coeffi- cient to provide the necessary temperature compensation. The parallel combination of R15 and R7 is provided to keep the sub tracter section’s gain for positive and negative inputs matched over temperature. Frequency compensation is provided by R11, R12, and C1 and C2. The bandwidth of the circuit is 300 kHz at input signals greater than 50 µA, and decreases smoothly with decreasing signal levels. To trim the circuit, set the input currents to 10 µA and trim A3’s offset using the amplifier’s trim potentiometer so the out- put equals 0. Then set I1 to 1 µA and adjust the output to equal 1 V by trimming R10. Additional offset trims on the amplifiers A1 and A2 can be used to increase the voltage input accuracy and dynamic range. The very low input current of the AD549 makes this circuit use- ful over a very wide range of signal currents. The total input current (which determines the low level accuracy of the circuit) is the sum of the amplifier input current, the leakage across the compensating capacitor (negligible if polystyrene or Teflon ca- pacitor is used), and the collector to collector, and collector to base leakages of one side of the dual log transistors. The magni- tude of these last two leakages depend on the amplifier’s input offset voltage and are typically less than 10 fA with 1 mV offsets. The low level accuracy is limited primarily by the amplifier’s in- put current, only 60 fA maximum when the AD549L is used. Figure 45. Log Ratio Amplifier The effects of the emitter resistance of Q1 and Q2 can degrade the circuit’s accuracy at input currents above 100 µA. The net- works composed of R13, D1, R16, and R14, D2, R17 compen- sate for these errors, so that this circuit has less than 1% log conformance error at 1 mA input currents. The correct value for R13 and R14 depends on the type of log transistors used. 49.9 k Ω resistors were chosen for use with LM394 transistors. Smaller resistance values will be needed for smaller log transistors. |
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