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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.
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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(AV(diff) / AV(cm)) • where AV(diff) is the voltage gain for differential signals and AV(cm) is the gain for common-mode signals. • A perfect diff-amp would have AV(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(AV(diff) / AV(cm)) = 20 Log(100 / 0.001) = 20 Log(100,000) = 20 Log(105) = 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(AV(diff) / AV(cm)) So AV(cm) = AV(diff) / Log-1(CMRR/20) = 10 / Log-1(80/20) = 10 / 104= 10-3 = 0.001 So at the output there will be 10 mV of signal and 1 mV of noise
Instrumentation Amps Except for Ri, all the above can be on one chip.
Instrumentation Amps Advantages of instrumentation amplifiers are: • Gain set by one resistor • High CMRR • High Zin on both input pins • Work well with most transducers
Transconductance Amps • Operational transconductance amplifiers (OTAs) look like other op-amps, but the output is a current instead of a voltage. • Gain is a transconductance (mutual-conductance) gm = iout / Vin • The value of gm is proportional to a DC bias current: gm = KIB • OTAs have relatively wide bandwidth. • OTAs have high output impedance (Zout). • The gain control by a current allows one signal to multiply another.
Optoisolators An LED and a phototransistor in one package current cannot pass from one side to the other.
Optoisolators Some important parameters: • Isolation voltage (typically thousands of Volts) • Current Transfer Ratio (CTR = IC/IF × 100%) • Speed (how fast can transistor turn on and off)
Voltage-Controlled Oscillators Output frequency is proportional to input voltage.
VCO Applications Some applications: • Frequency modulator • Adjustable carrier-oscillator for a radio transmitter • Adjustable signal source • Analog-to-digital converter • Building block for Phase-Locked Loops (PLLs)
Phase-Locked Loops Used in communications circuits.
PLLs • The VCO is set to run at a center frequency. • The VCO output is compared to the input in a phase detector circuit. The bigger the phase difference between the two frequencies, the higher the voltage out of the phase detector. • The output of the phase detector is fed through a LPF and becomes the control signal for the VCO. That closes the feedback loop. • The VCO will eventually “lock on” to the input signal and “track” it as the input frequency changes. The VCO frequency will match the input frequency.
PLL Frequency Synthesizer f(out) = (n2/n1 ) fXTAL