chapter 15 special ics. objectives describe and analyze: common mode vs. differential...
TRANSCRIPT
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CHAPTER 15
Special
ICs
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Objectives
Describe and Analyze:• Common Mode vs. Differential• Instrumentation Amps• Optoisolators• VCOs & PLLs• Other Special ICs
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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)
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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.
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Differential Amplifier
Resistances must be symmetric for a diff-amp.
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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).
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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.
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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.
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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
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Instrumentation Amps
Except for Ri, all the above can be on one chip.
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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
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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 = K IB
• OTAs have relatively wide bandwidth.
• OTAs have high output impedance (Zout).
• The gain control by a current allows one signal to multiply another.
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Optoisolators
An LED and a phototransistor in one package
current cannot pass from one side to the other.
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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)
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Voltage-Controlled Oscillators
Output frequency is proportional to input voltage.
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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)
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Phase-Locked Loops
Used in communications circuits.
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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.
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PLL as an FM Demodulator
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PLL Frequency Synthesizer
f(out) = (n2 / n1 ) fXTAL