[SI-LIST] Re: Measuring uV signals in PSRR test
- From: olaney@xxxxxxxx
- To: asif_surti@xxxxxxxxxxx
- Date: Fri, 4 Apr 2008 18:05:29 -0700
For frequencies this low, you can use classic instrumentation methods.
First up is the single point ground, or "holy point" which serves as your
ground reference. The regulator should pick up ground at this point and
nowhere else. Likewise, this is the connection point where the returning
load current meets the return lead from the power supply. Read up on
Kelvin connections to understand this better. The leads of your
measuring instrument should be bridged between the holy point and the
output pin of the regulator.
Although it is good practice to have an earth ground somewhere in the
setup, let only one device supply it, such as via the power supply line
cord. Other instruments should be floated from any ground connection
other than the holy point. You can use battery operation or a shielded
isolation power transformer to do this. If this is not sufficient, it
might help to inject AC onto the DC supply through a signal transformer
in series (it might take more than one to cover the entire frequency
range. Make sure they can handle the DC current).
Measure the input injection level independently of the output response
level. You can do one, and then the other with the same instrument or
use two independent instruments. Don't try to use two channels at once
on the same 'scope or meter. There are also refinements possible with a
differential probe. If you are intrepid enough to use a network analyzer
for this measurement (not always a good choice for this purpose), then
the use of signal transformers to create galvanic isolation for both
injection and response can do a lot to keep things sane and credible. To
calibrate, you directly connect these to each other (with the expected DC
current flowing through for best accuracy). Look for resonances and
avoid them -- change the setup for different frequency bands if you have
to.
To measure very large dB ratios it is often helpful to tack solder a
brass or copper shield between the input and output halves of the circuit
to avoid capacitive coupling that bridges around the device you are
measuring, though I don't think that will be required here.
Finally, see if you can make the measurement in a shielded room, or at
least put your test setup in a metal box or cage, use coax connectors at
the wall, and use shielded connections; the typical busy lab has
everything from fluorescent lights to AM radio stations to noisy
breadboards generating electromagnetic smog sufficient to swamp your
signal.
For very low response levels, you might need to add a preamp using these
principles.
The essence of the matter is to ensure that you are measuring the
response of the LDO alone, independent of voltage drops created in the
load and supply connections and anything else. Do your homework, and
best of success to you.
Orin Laney
On Fri, 4 Apr 2008 16:26:57 +0530 (IST) Asif Surti
<asif_surti@xxxxxxxxxxx> writes:
> Hi,
> Good morning! I have been measuring PSRR for LDOs. It is of the
> order of 70dB, so I need to measure AC signal of about 300 uV from
> 10Khz to 500Khz.
>
> What can be a good way to measure the low amplitude AC signals?
> Please can anyone help me in this.
>
> Thanks for help,
> Best Regards,
> Asif
>
>
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