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From: hal on 1 Aug 2010 11:29
Hello all,

I've been following this thread and it has brought up some issues I'm
currently working on. First of all, I'm a hobbyist that likes to learn by
doing; as such, I'm interested in making a circuit do what I want without
regards to a BOM - especially in regards to performance. In other words, I
like "best" rather than "cheapest" or "easiest."

Currently I'm working on a LCR meter for personal use with better than .1%
accuracy (closer to .02% if possible) for all functions. Also, all
calibration will be internal and automatic; I have quite a few of Vishay's
S102C series resistors (.01%, 2ppm) so I have a ready internal reference to
use, keeping all measurements ratiometric if possible, and use resistance as
the "known" in all equations.

I'm measuring capacitance using constant-current charge time, so I already
will know the capacitance before I measure the ESR and can account for Z. I
originally planned to use a peak-detection (measurement) scheme with a
100khz low distortion sine wave, however 1-2% accuracy was the best it
seemed that I could do (Yes, I realize that for ESR, that is good enough -
but remember, I'm doing this to learn and just to see if I can for the fun
of it). So, as it stands, I am planning on using a S/H or tracking ADC with
a high enough sample rate to detect both high and low peaks of both the DUT
and a reference resistance and calculate the ESR from there.

As the required additional circuitry to measure the ESR is almost as much as
all of the rest put together, I can't help but wonder if there's an easier
way that I've missed and I'm using a sledge hammer to drive a nail. So, I
would welcome any suggestions, tips or hints.

Thanks,

Hal
From: Jim Thompson on 1 Aug 2010 11:45
On Sun, 1 Aug 2010 15:29:19 GMT, hal(a)nospam.com wrote:

>Hello all,
>
>I've been following this thread and it has brought up some issues I'm
>currently working on. First of all, I'm a hobbyist that likes to learn by
>doing; as such, I'm interested in making a circuit do what I want without
>regards to a BOM - especially in regards to performance. In other words, I
>like "best" rather than "cheapest" or "easiest."
>
>Currently I'm working on a LCR meter for personal use with better than .1%
>accuracy (closer to .02% if possible) for all functions. Also, all
>calibration will be internal and automatic; I have quite a few of Vishay's
>S102C series resistors (.01%, 2ppm) so I have a ready internal reference to
>use, keeping all measurements ratiometric if possible, and use resistance as
>the "known" in all equations.
>
>I'm measuring capacitance using constant-current charge time, so I already
>will know the capacitance

Are you not deluding yourself that you know "constant-current" to 0.1%
accuracy?

>before I measure the ESR and can account for Z. I
>originally planned to use a peak-detection (measurement) scheme with a
>100khz low distortion sine wave, however 1-2% accuracy was the best it
>seemed that I could do (Yes, I realize that for ESR, that is good enough -
>but remember, I'm doing this to learn and just to see if I can for the fun
>of it). So, as it stands, I am planning on using a S/H or tracking ADC with
>a high enough sample rate to detect both high and low peaks of both the DUT
>and a reference resistance and calculate the ESR from there.
>
>As the required additional circuitry to measure the ESR is almost as much as
>all of the rest put together, I can't help but wonder if there's an easier
>way that I've missed and I'm using a sledge hammer to drive a nail. So, I
>would welcome any suggestions, tips or hints.
>
>Thanks,
>
>Hal

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

Spice is like a sports car...
Performance only as good as the person behind the wheel.
From: hal on 1 Aug 2010 12:00

On 1-Aug-2010, Jim Thompson
<To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote:

> On Sun, 1 Aug 2010 15:29:19 GMT, hal(a)nospam.com wrote:
>
> >Hello all,
> >
> >I've been following this thread and it has brought up some issues I'm
> >currently working on. First of all, I'm a hobbyist that likes to learn
> >by
> >doing; as such, I'm interested in making a circuit do what I want without
> >regards to a BOM - especially in regards to performance. In other words,
> >I
> >like "best" rather than "cheapest" or "easiest."
> >
> >Currently I'm working on a LCR meter for personal use with better than
> >.1%
> >accuracy (closer to .02% if possible) for all functions. Also, all
> >calibration will be internal and automatic; I have quite a few of
> >Vishay's
> >S102C series resistors (.01%, 2ppm) so I have a ready internal reference
> >to
> >use, keeping all measurements ratiometric if possible, and use resistance
> >as
> >the "known" in all equations.
> >
> >I'm measuring capacitance using constant-current charge time, so I
> >already
> >will know the capacitance
>
> Are you not deluding yourself that you know "constant-current" to 0.1%
> accuracy?

Probably so - hence my request for suggestions. I suppose that I should
have emphasized that this is not only something that I do for enjoyment, but
to _learn_. As such, I would welcome feedback on not only that I *am* going
wrong, but *how* as well.


>
> >before I measure the ESR and can account for Z. I
> >originally planned to use a peak-detection (measurement) scheme with a
> >100khz low distortion sine wave, however 1-2% accuracy was the best it
> >seemed that I could do (Yes, I realize that for ESR, that is good enough
> >-
> >but remember, I'm doing this to learn and just to see if I can for the
> >fun
> >of it). So, as it stands, I am planning on using a S/H or tracking ADC
> >with
> >a high enough sample rate to detect both high and low peaks of both the
> >DUT
> >and a reference resistance and calculate the ESR from there.
> >
> >As the required additional circuitry to measure the ESR is almost as much
> >as
> >all of the rest put together, I can't help but wonder if there's an
> >easier
> >way that I've missed and I'm using a sledge hammer to drive a nail. So,
> >I
> >would welcome any suggestions, tips or hints.
> >
> >Thanks,
> >
> >Hal
>
> ...Jim Thompson
> --
From: Jim Thompson on 1 Aug 2010 12:17
On Sun, 1 Aug 2010 16:00:54 GMT, hal(a)nospam.com wrote:

>
>On 1-Aug-2010, Jim Thompson
><To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote:
>
>> On Sun, 1 Aug 2010 15:29:19 GMT, hal(a)nospam.com wrote:
>>
>> >Hello all,
>> >
>> >I've been following this thread and it has brought up some issues I'm
>> >currently working on. First of all, I'm a hobbyist that likes to learn
>> >by
>> >doing; as such, I'm interested in making a circuit do what I want without
>> >regards to a BOM - especially in regards to performance. In other words,
>> >I
>> >like "best" rather than "cheapest" or "easiest."
>> >
>> >Currently I'm working on a LCR meter for personal use with better than
>> >.1%
>> >accuracy (closer to .02% if possible) for all functions. Also, all
>> >calibration will be internal and automatic; I have quite a few of
>> >Vishay's
>> >S102C series resistors (.01%, 2ppm) so I have a ready internal reference
>> >to
>> >use, keeping all measurements ratiometric if possible, and use resistance
>> >as
>> >the "known" in all equations.
>> >
>> >I'm measuring capacitance using constant-current charge time, so I
>> >already
>> >will know the capacitance
>>
>> Are you not deluding yourself that you know "constant-current" to 0.1%
>> accuracy?
>
>Probably so - hence my request for suggestions. I suppose that I should
>have emphasized that this is not only something that I do for enjoyment, but
>to _learn_. As such, I would welcome feedback on not only that I *am* going
>wrong, but *how* as well.

Yep. A welcome change in SED direction.

Pondering that, it's going to be tricky to make a loop-stabilized
current source that tracks a slewing capacitor's change in voltage
(Early-effect or channel-length modulation).

Perhaps _not_ use a classic current source, instead sense capacitor
current (into a virtual ground) and adjust a voltage drive?

I presume you're measuring time-to-voltage to get capacitance?

>
>
>>
>> >before I measure the ESR and can account for Z. I
>> >originally planned to use a peak-detection (measurement) scheme with a
>> >100khz low distortion sine wave, however 1-2% accuracy was the best it
>> >seemed that I could do (Yes, I realize that for ESR, that is good enough
>> >-
>> >but remember, I'm doing this to learn and just to see if I can for the
>> >fun
>> >of it). So, as it stands, I am planning on using a S/H or tracking ADC
>> >with
>> >a high enough sample rate to detect both high and low peaks of both the
>> >DUT
>> >and a reference resistance and calculate the ESR from there.
>> >
>> >As the required additional circuitry to measure the ESR is almost as much
>> >as
>> >all of the rest put together, I can't help but wonder if there's an
>> >easier
>> >way that I've missed and I'm using a sledge hammer to drive a nail. So,
>> >I
>> >would welcome any suggestions, tips or hints.
>> >
>> >Thanks,
>> >
>> >Hal
>>
>> ...Jim Thompson
>> --

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

Spice is like a sports car...
Performance only as good as the person behind the wheel.
From: John Larkin on 1 Aug 2010 12:54
On Sun, 1 Aug 2010 15:29:19 GMT, hal(a)nospam.com wrote:

>Hello all,
>
>I've been following this thread and it has brought up some issues I'm
>currently working on. First of all, I'm a hobbyist that likes to learn by
>doing; as such, I'm interested in making a circuit do what I want without
>regards to a BOM - especially in regards to performance. In other words, I
>like "best" rather than "cheapest" or "easiest."
>
>Currently I'm working on a LCR meter for personal use with better than .1%
>accuracy (closer to .02% if possible) for all functions. Also, all
>calibration will be internal and automatic; I have quite a few of Vishay's
>S102C series resistors (.01%, 2ppm) so I have a ready internal reference to
>use, keeping all measurements ratiometric if possible, and use resistance as
>the "known" in all equations.
>
>I'm measuring capacitance using constant-current charge time, so I already
>will know the capacitance before I measure the ESR and can account for Z. I
>originally planned to use a peak-detection (measurement) scheme with a
>100khz low distortion sine wave, however 1-2% accuracy was the best it
>seemed that I could do (Yes, I realize that for ESR, that is good enough -
>but remember, I'm doing this to learn and just to see if I can for the fun
>of it). So, as it stands, I am planning on using a S/H or tracking ADC with
>a high enough sample rate to detect both high and low peaks of both the DUT
>and a reference resistance and calculate the ESR from there.
>
>As the required additional circuitry to measure the ESR is almost as much as
>all of the rest put together, I can't help but wonder if there's an easier
>way that I've missed and I'm using a sledge hammer to drive a nail. So, I
>would welcome any suggestions, tips or hints.
>
>Thanks,
>
>Hal

Really precise capacitance measurement (and "really precise" starts at
1% maybe) generally uses a bridge driven by a sine wave. One problem
with caps is that C varies with frequency because of dielectric
absorption and other effects, so the frequency is usually specified
for accurate measurement. DA can make a ramp-type C measurement
nonlinear.

If you have a fast ADC, I'd suggest just taking a lot of samples and
absolute value+averaging. That would be better, lower noise data than
looking for/at the peak, and you could include a lot more samples.

If you can use a capacitor for the reference, you can make a voltage
divider with Cref and Cx, digitize the overall voltage and the
midpoint voltage, and do the math. That's ratiometric on the
excitation source, and works for resistors, too. I guess you could
make a divider from a precision resistor and Cx, given a good sine
reference.

A decent 16-bit SAR ADC is cheap nowadays and they tend to be
amazingly linear. Sampling all up and down the waveform makes it even
better, and averaging lots of samples can extend the resolution beyond
16 bits. You can do synchronous detection tricks to reduce noise. The
nice thing about an ADC approach is that the tweaks are software.

To do fancy stuff, like 3/4 terminal measurements, you might buy a
couple of old HP or GR RLC bridge manuals and study their topologies.
Those old manuals generally had schematics and good theory sections.


John

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