Subject:
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Re: HandyBoard A/D converter?
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Newsgroups:
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lugnet.robotics.handyboard
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Date:
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Sat, 26 Feb 2000 18:54:02 GMT
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Viewed:
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1065 times
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The 68HC11 has references for the range of the A/D converter. In the HB
design they are hardwired to 0 and +5. I believe the pin is Vrh at
<http://el.www.media.mit.edu/groups/el/projects/handy-board/schemv12/hbsch12
.pdf> and is currently wired to +5.
--chris prosser
From the motorola manual:
2.2.7 A/D Reference and Port E Pins (V REFL , V REFH , PE[7:0])
The V REFH and V REFL pins provide the reference voltages for the A/D
converter cir-cuitry.
Since the A/D converter is an all-capacitive charge-redistribution
converter,
there is essentially no dc current associated with these pins. Very small
dynamic cur-rents
are caused by charge-redistribution switching during conversions (see
SEC-TION
12 ANALOG-TO-DIGITAL CONVERTER SYSTEM). These pins are normally
connected to V DD and V SS through a low-pass filter network (see Figure
2-13) to iso-late
noise on the logic power supply from the relatively sensitive analog
measure-ments.
A low-noise precision reference supply can alternatively be used. There
should
be at least 2.5 V between V REFL and V REFH for full A/D accuracy. Lower
values will
result in more inaccuracy, but the converter will continue to operate. The
A/D system
is tested at 4.5 V and 5.5 V across the reference supply pins.
There is an inherent diode from V REFL to V SS . If V REFL goes below V SS
by more than
this diode drop, any conversion in progress may be corrupted, but no
permanent phys-ical
damage will result until significant current is drawn. The only documented
cases
of damage have been caused by blatant misapplication, such as connecting -12
V di-rectly
to the V REFL pin. Since no P-channel devices are associated with the V REFH
pin,
there is no diode clamping to V DD . The gates of analog switches associated
with the
A/D reference and input pins are controlled by signals that switch between V
SS and
about 7 V. This higher-than-V DD supply is the output of a charge pump
(separate from
the charge pump used for programming on-chip EEPROM). There is no special
re-quirement
to keep V REFH below V DD . In fact, the converter will continue to produce
good results up to approximately 6 V on V REFH .
----- Original Message -----
From: "Barrus, Brad" <handyboard@media.mit.edu>
To: <lugnet.robotics.handyboard@lugnet.com>
Sent: Saturday, February 26, 2000 7:22 AM
Subject: HandyBoard A/D converter?
> Dear Handy Board Masters,
> I am trying to use the handy board for some A/D conversions. The
> converter is set up on a 5 volt scale, the converter is an eight bit
> converter. Over this range I get a resolution of 19.5mV per Step. My
> application is needed from 0 to 600mV. At the current settings this would • be
> terrible resolution. Here is my question. Is there any way to set the
> Voltage reference from 5Volts to 600mV. This would allow me to use the
> entire range of the A/D converter and would give me very good resolution.
> Any response would be appreciated. Thank you Brad
>
>
> <http://www.cabletron.com/>
> Brad Barrus
> SLC Hardware Eng.
> bbarrus@cabletron.com <mailto:bbarrus@cabletron.com>
> Phone: 801-887-9490 Fax: 801-887-5789 2835 South Decker Lake Drive Salt • Lake
> City, UT 84119
>
>
>
>
>
>
> --
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Message is in Reply To:
 | | HandyBoard A/D converter?
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| Dear Handy Board Masters, I am trying to use the handy board for some A/D conversions. The converter is set up on a 5 volt scale, the converter is an eight bit converter. Over this range I get a resolution of 19.5mV per Step. My application is (...) (25 years ago, 26-Feb-00, to lugnet.robotics.handyboard)
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