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Martin Shepherd wrote:<br>
<blockquote type="cite"
cite="midPine.LNX.4.58.0401021702040.9908@goblin.caltech.edu">
<pre wrap="">While in the process of updating my CCB external interfaces document
to reflect changes that have been proposed in subsequent discussions,
I encountered a problem. Randy's suggestion of using a single power
supply for the AD8138 in the CCB, instead of the dual +5V and -5V
supplies that I originally proposed, seemed like a good one,
especially wrt being able to get rid of the ADC clamping diodes and
some associated liabilities. Unfortunately, it turns out that this is
potentially problematic, because in single supply mode, the AD8138
data-sheet says that its DC input voltages can only swing between 1V
and 3.8V. While in theory this is enough to accomodate the 2.5V swing
arriving on each line of each of the differential pairs from the
receiver, to do so would require that the common mode voltage arriving
at the inputs to the CCB be 2.4V +/- 0.15V, whereas the common-mode
voltage following the matching resistors at the outputs of the
receiver, in the currently proposed video driver circuit, is just
1.25V. Given that before these matching resistors, the common-mode
output voltage is double that seen after the matching resistors, to
present the correct 2.4V common-mode voltage at the input to the CCB,
would require increasing the common-mode output of the AD8138 driver
in the receiver to 4.8V, while maintaining the 2.5V swing of each
output above and below this voltage. In the current video driver
circuit this isn't possible, because the +5V supply limits the output
of the AD8138 to less than 5V.
Having just updated the power-management circuit, the analog input
circuit, power-connector pinout diagram, and the text of my document
to accomodate the removal of the -5V supply, among other changes, it
would be great if we can find a way to resolve this problem without
having to revert to using dual +/-5V analog power-supplies in the CCB.
One possibility would be to offset the signal inputs of the AD8138 in
the receiver, to make them always positive. This would allow the
receiver AD8138s to be powered from a single positive power-supply.
According to the data-sheet, in this mode the power-supply voltage can
be up to 11V, which would be enough to accomodate the required 4.8V
common-mode output voltage.
This also brings up a question about clamping. In Randy's circuit, the
outputs of the receiver AD8138s are clamped at the input to the
cables. Since the parts that will break if the input voltages go out
of range, are the AD8138s in the CCB, doesn't it make more sense to
place clamping diodes at the CCB inputs than the receiver outputs?
Also, since I have brought up the subject of power-supplies in the
CCB, is NRAO or Caltech now responsible for the power-management
circuit, now that the responsibility for the main PCB has changed
hands from Caltech to NRAO?
Martin
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</pre>
</blockquote>
Martin,<br>
<br>
As you've no doubt realized by now, correctly interpreting the AD8138's<br>
data sheet can be quite a chore! I suggest that you revisit ADI's data
sheet <br>
(rev E) and consider the following...<br>
<br>
Referring to page 3 (which pertains to single +5 volt operation), the
numbers <br>
you quote regarding the input voltage range (1.0 to 3.8 volts) apply to
the <br>
Vocm input, NOT the +/- Din inputs. Note that this page is organized
under <br>
two main headings. The first section titled "+/- Din to +/- OUT
Specifications"<br>
pertains to the +/- Din inputs, while the second section titled "Vocm
to +/-OUT<br>
Specifications" pertains only to the Vocm input. In carefully
reviewing this<br>
particular section, you'll see that ADI goes on to specify the "Input
Resistance" <br>
to be only 100K ohms and the "Gain" to be approximately 1. Therefore,
the<br>
data appearing in this section MUST pertain only to the Vocm input
pin...<br>
<br>
I refer you also to the same data sheet, page 13 under the section
titled<br>
"3 V OPERATION". Here, the third paragraph explains that, even with<br>
its negative supply pin at ground potential, the AD8138 is capable of<br>
level shifting a single-ended, BIPOLAR input (i.e., an input which
swings<br>
symmetrically above and below ground). From this, one would infer<br>
(correctly) that a differential amplifier built with an AD8138,
operating <br>
from a single +5 volt supply could easily handle the signal ranges in<br>
question (i.e., +0.625 volts to +1.875 volts with a common mode of<br>
1.25 volts) at both of its inputs. (By the way, If these levels look
strange <br>
to you, consider this... The receiver amp's AD8138 output is 5 volts <br>
(differential) impressed upon a common mode of 2.5 volts. Thus, the <br>
AD8138's "non-inverted" output swings from +1.25 volts to +3.75 volts, <br>
while its "inverted" output swings from +3.75 volts to +1.25 volts.
These<br>
levels are then divided by two through the matching resistors installed
at <br>
both ends of the cable.)<br>
<br>
Therefore, I still contend that operating the CCB's AD8138's with a <br>
single +5 volt supply will present no problems. And, in light of this,
it <br>
would seem that your other concerns regarding the use of +/- 5 volt<br>
supplies vs. using a single + 5 volt supply may have just gone away...<br>
<br>
Now, regarding your comments about the location of the clamping
diodes...<br>
<br>
My concept of the system as a whole calls for clamping diodes at BOTH<br>
ends. Since your original document calls for the ability to freely
swap cables,<br>
etc., it seems only prudent to do so. Indeed, it's good engineering
practice<br>
to provide diode clamps and current limiting resistors on ANY analog
input<br>
OR output which sees the real world through a readily connected cable.<br>
<br>
So...<br>
<br>
The diodes installed at the receiver end are there to protect the
receiver, <br>
while those (which should be) installed at the CCB end are there to
protect <br>
the CCB...<br>
<br>
Randy<br>
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