AN331

4

Rev. 0.1

A commercially available phase gain meter optimized for power supply analysis is available from Venable Inc. The

Venable meter can be used with an injection transformer from Venable, or an ordinary transformer can be used as

long as it keeps a low ac and dc impedance.

Good results have also been achieved with a Bode 100 phase gain meter from Omicron Labs and using Coilcraft

BU15-7521ROBL common mode choke hooked up as a transformer (input on 1,2 output on 3,4). When terminated

with a 50

Ω resistor on the input side, this transformer gives <100 Ω impedance on the output side from dc to well

over 10 MHz insuring that the transformer itself does not impact the feedback loop.

Other phase gain meters can be used as long as they are capable of operating in the 100 Hz to 40 kHz range of

interest and have a provision for high impedance (1 M

Ω) probes.

The phase-gain meter is used to measure the output voltage response to the input voltage that is stimulated by the

critical. It should be large enough that the signals can be measured but not so large that there is distortion. In

practice, a signal level of –20 to –30 dBm (50

Ω reference) has been found to be satisfactory.

The magnitude and phase of the ratio of the output voltage the input voltage is what needs to be measured to

examine loop stability. In general it is good practice to have at least 50 degrees of phase margin when the gain is

unity (zero dB) and 10 dB of gain margin when the phase reaches zero degrees.

Figure 3. Phase and Gain of VOUT with Respect to VIN

Experimental results for the standard isolated and non-isolated reference designs are shown in Figures 4 and 5

(see also AN296).