MT TUTORIAL Op Amp Power Supply Rejection R atio PSRR and Supply Voltages POWER SUPPLY REJECTION RATIO PSRR If the supply of an op amp changes its output should not but it ty pically does

MT TUTORIAL Op Amp Power Supply Rejection R atio PSRR and Supply Voltages POWER SUPPLY REJECTION RATIO PSRR If the supply of an op amp changes its output should not but it ty pically does - Description

If a change of X volts in the supply produces an output voltage ch ange of Y volts then the PSRR on that supply referred to the output RTO is XY The dime nsionless ratio is generally called the power supply rejection ratio PSRR and Power Supply Reje ID: 26585 Download Pdf

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MT TUTORIAL Op Amp Power Supply Rejection R atio PSRR and Supply Voltages POWER SUPPLY REJECTION RATIO PSRR If the supply of an op amp changes its output should not but it ty pically does

If a change of X volts in the supply produces an output voltage ch ange of Y volts then the PSRR on that supply referred to the output RTO is XY The dime nsionless ratio is generally called the power supply rejection ratio PSRR and Power Supply Reje

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MT TUTORIAL Op Amp Power Supply Rejection R atio PSRR and Supply Voltages POWER SUPPLY REJECTION RATIO PSRR If the supply of an op amp changes its output should not but it ty pically does




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Presentation on theme: "MT TUTORIAL Op Amp Power Supply Rejection R atio PSRR and Supply Voltages POWER SUPPLY REJECTION RATIO PSRR If the supply of an op amp changes its output should not but it ty pically does"— Presentation transcript:


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MT-043 TUTORIAL Op Amp Power Supply Rejection R atio (PSRR) and Supply Voltages POWER SUPPLY REJECTION RATIO (PSRR) If the supply of an op amp changes, its output should not, but it ty pically does. If a change of X volts in the supply produces an output voltage ch ange of Y volts, then the PSRR on that supply (referred to the output, RTO) is X/Y. The dime nsionless ratio is generally called the power supply rejection ratio (PSRR), and Power Supply Rejection (PSR ) if it is expressed in dB. However, PSRR and PSR are almost always used interchangeably, and there is little

standardization within the semiconductor industry. . PSRR or PSR can be referred either to the outpu t (RTO) or the input (RTI). The RTI value can be obtained by dividing the RTO value by the amplif ier gain. In the case of the traditional op amp, this would be the noise gai n. The data sheet shou ld be read carefully, because PSR can be expressed either as an RTO or RTI value. PSR can be expressed as a positive or negative value in dB, depending on whether the PSRR is defined as the power supply change divided by the output voltage ch ange, or vice-versa. There is no accepted standard for

this in the industry, and bot h conventions are in use. If the amplifier has dual supplies, it is customary to express PSR se parately for each. This is very useful for amplifiers that can be used in eith er dual or single-supply applications. It is extremely important to remember that PS R is very much a functi on of ripple or noise frequency as shown in the plot for the OP1177 op amp. In most cases, the corner frequency of the roll-off follows that of the open-loop gain, a nd the slope is approximately 6 dB per octave (20 dB per decade). Typical PSR for the OP1177 is shown in Figure 1 below.

SS SS Figure 1: OP1177 Power S upply Rejection (PSR) Rev.0, 10/08, WK Page 1 of 3
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MT-043 A test setup used to measure PSRR is shown in Figur e 2 below. Note that it is similar to the test setup used to measure CMRR (see Tutorial MT-042 ). DUT OUT A1 20k 100 100 20k 10k 10k 0V +14V 15V 14V +15V S1 S2 PSRR (RTI) = 101 1V OUT A1: HIGH GAIN, LOW V OS , LOW I Figure 2: Test Setup for Measuring Po wer Supply Rejecti on Ratio (PSRR) The voltages are chosen for a sy mmetrical power supply change of 1 V. Other values may be used where appropriate, and the measurement can be made for

the positive and negative supply separately. POWER SUPPLIES AND POWER DISSIPATION Op amps have no ground terminal. Specifications for the power supply are quite often in the form X volts, but in fact it might equally be ex pressed as 2X volts. What is important is where the CM and output ranges lie relative to the supplies. This information may be provided in tabular form or as a graph. Often data sheets will advise that an op amp w ill work over a range of supplies (from +3 V to 16.5 V for example), and will then give parameters at several values of supply, so that users may extrapolate. If

the minimum supply voltage is quite high, it is usually because the device uses a structure requiring a threshold voltage to func tion (e.g., zener diode). Data sheets also give current consumption. Any current flowing into one supply pin will flow out of the other or out of the output terminal. When the output is open circuit, the dissipation is easily calculated from the supply voltage and current. When current flows into a load, it is easiest to calculate the total dissipation (r emember that if the lo ad is grounded to the center rail the load current flows from a supply to ground, not

betw een supplies), and then subtract the load dissipation to obtain the device dissipation. Data sheets normally give details of thermal resistances and maximum junctio n temperature ratings, from wh ich dissipation limits may be Page 2 of 3
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MT-043 calculated knowing conditions. Details of furthe r considerations relatin g to power dissipation, heatsinking, etc., can be found in Chapter 7 of Reference 1. POWER SUPPLIES AND DECOUPLING Because op amp PSRR is frequency dependent, op amp power supplies must be well decoupled. At low frequencies, several devices may share a 10-50

F capacitor on each supply, provided it is no more than 10 cm (PC track distance) from any of them. C1 C2 C3 C4 +V V LARGE AREA GROUND PLANE LEAD LENGTH MINIMUM C1, C2: LOCALIZED HF DECOUPLING, LOW INDUCTANCE CERAMIC, 0.1F C3, C4: SHARED LF DECOUPLING, ELECTROLYTIC, 10 TO 50F < 10cm < 10cm Figure 3: Proper Low and High-Frequen cy Decoupling Techniques for Op Amps At high frequencies, each IC should have the supply leads decoupled by a low inductance 0.1 F (or so) capacitor with short leads/PC tracks. Thes e capacitors must also provide a return path for HF currents in the op amp load.

Typical decoupling circuits are s hown in Figure 3 above. Further bypassing and decoupling information can be found in the last chapter of References 1 and 2. REFERENCES: 1. Hank Zumbahlen, Basic Linear Design , Analog Devices, 2006, ISBN: 0-915550-28-1. Also available as Linear Circuit Design Handbook , Elsevier-Newnes, 2008, ISBN-10: 0750687037, ISBN-13: 978- 0750687034. Chapter 1 2. Walter G. Jung, Op Amp Applications , Analog Devices, 2002, ISBN 0-916550-26-5, Also available as Op Amp Applications Handbook , Elsevier/Newnes, 2005, ISBN 0-7506-7844-5. Chapter 1. Copyright 2009, Analog

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