9/26 10.2.5 Transconductance and Transimpedance Amplifiers When the input is a voltage and the output is a current,the amplifier is called transconductance whereas the reverse situation is called transimpedance. 60 MHz 2000 V/us ◆Transconductance: Monolithic Op Amp AD844 -A single-chip solution for converting voltage into a current output,can be used as voltage controlled current source. Ground Base ● ● 9 ◆Transimpedance: 2o Emitter -A single-chip solution for converting control Switching Stage photodiode current into a differential OTA voltage output,supporting data rates from Sampling Comparator(SC) 12Collector 155Mbps to 11.1Gbps. (louT） -Fiber Optic Transimpedance Amplifiers o19 SOTA Biasing S/H 11 O lo Adjust Photo Detector with integrated loss of In- signal (LOS)detection circuitry. OPA615 13 5 cc cc 20/26 10.2.6 Noise Problems,Shielding,and Grounding Amplifier drifts can be controlled when line voltage and temperature are kept constant. Inductive pickup (via magnetic field),electrostatic pickup (via stray capacitance),and ground loops can cause large error voltages,often concentrated at the power-line frequency. ◆Solutions: Remove interfering devices (power lines,motors,transformers,fluorescent() lamps,relays,etc.)from the near field of sensitive signal circuits. Use of an enclosing conductive shield as shown in Fig.10.13.The captured charges in she shield must be drained off to a satisfactory ground. Electrostatic source Use twisted pair of conductors to reduce the pick up of low frequency magnetically induce noise. Source-to-shield capacitance Signal conductors -Shield Fig.10.13 Electrostatic Shielding Shield-to-cable Ground capacitance‹ When the input is a voltage and the output is a current, the amplifier is called transconductance whereas the reverse situation is called transimpedance. ‹ Transconductance: − A single-chip solution for converting voltage into a current output, can be used as voltage controlled current source. ‹ Transimpedance: − A single-chip solution for converting photodiode current into a differential voltage output, supporting data rates from 155Mbps to 11.1Gbps. − Fiber Optic Transimpedance Amplifiers − Photo Detector with integrated loss of signal (LOS) detection circuitry. 10.2.5 Transconductance and Transimpedance Amplifiers 60 MHz 2000 V/μs Monolithic Op Amp AD844 19/26 ‹ Amplifier drifts can be controlled when line voltage and temperature are kept constant. ‹ Inductive pickup (via magnetic field), electrostatic pickup (via stray capacitance), and ground loops can cause large error voltages, often concentrated at the power-line frequency. ‹ Solutions: − Remove interfering devices (power lines, motors, transformers, fluorescent(荧光) lamps, relays, etc.) from the near field of sensitive signal circuits. − Use of an enclosing conductive shield as shown in Fig. 10.13. The captured charges in she shield must be drained off to a satisfactory ground. 10.2.6 Noise Problems, Shielding, and Grounding Fig.10.13 Electrostatic Shielding − Use twisted pair of conductors to reduce the pick up of low frequency magnetically induce noise. 20/26