CHAPTER 15: Special ICs ppt

CHAPTER 15 Special ICs Objectives Describe and Analyze: • Common Mode vs. Differential • Instrumentation Amps • Optoisolators • VCOs & PLLs • Other Special ICs Introduction • This chapter examines some important op-amp related topics such as common-mode rejection. • It also examines some non op-amp linear circuits such as Voltage Controlled Oscillators (VCOs) and Phase-Locked Loops (PLLs) Single-Ended vs. Differential A signal applied between an input and ground is called a single-ended signal. A signal applied from one input to the other input is called a differential signal. Differential Amplifier Resistances must be symmetric for a diff-amp. Common-Mode Signals • Ground-referenced signals applied simultaneously to both inputs of a diff-amp are common-mode signals. • Electrical noise and interference often appear as common-mode signals. • Signals from transducers are usually differential. • To extract small differential signals out of a “soup” of common-mode noise, a diff-amp requires a high common-mode rejection ratio (CMRR). Definition of CMRR • The common-mode rejection ratio (CMRR) of a diff- amp is defined as: CMRR = 20 Log(A V(diff) / A V(cm) ) • where A V(diff) is the voltage gain for differential signals and A V(cm) is the gain for common-mode signals. • A perfect diff-amp would have A V(cm) equal to zero, so it would have infinite CMRR. • Real diff-amps have CMRRs in the range of 90 dB to 110 dB or better. Example Calculation 1 • Find the CMRR required so that differential signals have a gain of 100 and common-mode signals have a gain of 0.001 (an attenuation) CMRR = 20 Log(A V(diff) / A V(cm) ) = 20 Log(100 / 0.001) = 20 Log(100,000) = 20 Log(10 5 ) = 20  5 = 100 dB CMRR is less if the external resistors are not matched. Example Calculation 2 • A diff-amp has a gain of 10 and a CMRR of 80 dB. The input is a differential signal of 1 mV on top of 1 Volt of common-noise. How much signal voltage, and how much noise voltage, will be at the output of the diff-amp? CMRR = 20 Log(A V(diff) / A V(cm) ) So A V(cm) = A V(diff) / Log -1 (CMRR/20) = 10 / Log -1 (80/20) = 10 / 10 4 = 10 -3 = 0.001 So at the output there will be 10 mV of signal and 1 mV of noise Instrumentation Amps Except for R i , all the above can be on one chip. . CHAPTER 15 Special ICs Objectives Describe and Analyze: • Common Mode vs. Differential • Instrumentation Amps • Optoisolators • VCOs & PLLs • Other Special ICs Introduction. Optoisolators • VCOs & PLLs • Other Special ICs Introduction • This chapter examines some important op-amp related topics such as common-mode rejection. • It also examines some non op-amp