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PSRR testing with analysers sans dc-bias ports

20 May 2016  | David Karpaty

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An amplifier's power-supply-rejection ratio (PSRR) is among the most commonly characterized parameters when analysing the performance of an op amp. Examples of some noise sources on an amplifier's power-supply pins include parasitic supply-line traces, their interaction with currents that the amplifier draws, and the noise that switching circuits sharing the same supply create. Both sources produce voltage-amplitude variations reproduced as noise signals at the amplifier's input pins.

Characterising PSRR over frequency commonly involves the use of analysers equipped with a dc-bias port, such as Agilent's 8753. To measure negative PSRR, for example, the amplifier's –VS pin comes through Port 1, with the negative dc voltage through the bias port, of the 8753 with a superimposed sinusoid. To complete the measurement, you measure the amplifier's output on Port 2. Unfortunately, the 8753 doesn't measure frequencies below 30kHz because of the limitations of the analyser's internal bias, T. Additionally, most PSRR-versus-frequency plots begin at frequencies far below 30kHz.

An alternative technique would involve the use of an analyser that has no dc-bias port but that can characterise frequency response as low as 10 or even 1Hz. One such analyser is the Stanford Research Systems SR785, which can make measurements better than –120 dB. One way of approaching this problem is to connect the output port of the SR785 to a buffer/inverting-summer circuit constructed with an Analog Devices AD8034.

Figure 1 illustrates a negative-PSRR test-circuit configuration. Pin 3 connects to the SR785 source-output port. Pin 1, which is VOUT of the buffer amplifier, connects to the reference port of the SR785. Here, the first amp isolates the output port of the SR785 from the dc bias and provides the sinusoidal output. The second amplifier within the AD8034 sums the dc bias and sinusoid, which it uses to feed the DUT's (device under test's) negative-supply pin. The figure omits all bypass capacitors at the DUT's negative-supply pin. A 1-kΩ resistor from Pin 3 to ground prevents the noninverting input from floating. The positive terminal of the external dc-power supply feeds Pin 6 through a 1-kΩ resistor. Connecting the DUT's output to Channel 2A of the SR785 completes the test-circuit configuration.

Figure 1: The AD8034 sums the analyzer's reference output and a dc bias, supplying the negative-supply voltage for the device under test to perform negative-PSRR analysis.

Building the buffer/inverting summer with an AD8034 dual amplifier is a good choice because it has a supply range of 5 to 24V; a signal-frequency response well beyond 1MHz; and a large capacitive-load-drive capability, allowing you to neglect the capacitance of test cables. Further, the AD8034 can deliver as much as 40 mA of load current.

Figure 2: You can neglect any loss the AD8034 incurs. The response of the AD8034 buffer/inverting summer from 10 Hz to 10 kHz is approximately flat.

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