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ccoats
18 Posts |
 Posted - 03 Aug 2004 : 17:29:23
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| We have both AGX and VGX boards. In section 6.2.6 under note 25 in both users manuals it states that there is a voltage divider with a series impedance of 43.2K ohms. When I measure the A/D inputs on the AGX I see 43.2K. When I measure it on the VGX I get ~1K. Why?? |
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vta
14 Posts |
Posted - 13 Sep 2005 : 15:22:03
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| On the AGX, only pins 5 through 7 have the voltage divider. The other 5 pins have no divider. Is there really supposed to be a divider on the VGX pins you checked? I don't own a VGX or I would check the manual. |
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ccoats
18 Posts |
Posted - 13 Sep 2005 : 16:37:29
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| It turns out that there is an extra suffing option that connects these analog input to the SM501 controller. R628-R631. With these removed the circuit behaves as expected and the 43.2k impedance is there. There is likely some clamping diodes in the SM501 not allowing the full analog voltage and loading the input source. Protection diodes usally are not all that accurate so difffrent loads appear which translates into diffrent voltage readings between units. These resistors are likely used for a zoomed video input port. |
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akidder
1519 Posts |
Posted - 13 Sep 2005 : 16:54:08
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Vta is correct. On the AGX and VGX, ADSmartIO pins PA5-PA7 provide the analog inputs. These inputs have a 43.2k input impedance.
The 1k input impedance that ccoats measured was from a part stuffing error on the VGX that connected AIN1-3 to VPDATA9-11.
[Thanks, ccoats, for the details!] |
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innomatix
12 Posts |
Posted - 01 Jun 2009 : 12:55:37
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| We are trying to expand the input range for the analog inputs on the VGX (to a max of about 30V). We are only using PA5-PA7. The values for the current divider circuit behave as expected (33.2K with 10K to ground), but if we place any additional resistance in-line with the input to change the divider, we get unpredictable results. Is it possible to see the schematic of the analog inputs to make sure we aren't missing anything else? Has anyone else had any luck making such modifiations? |
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cbadr
154 Posts |
Posted - 02 Jun 2009 : 13:39:37
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| Can you please clarify what resistance values you added? |
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cbadr
154 Posts |
Posted - 03 Jun 2009 : 11:51:23
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Here are some additional details that may help you with your design:
If you have a voltage divider at your input on J8, keep in mind that your lower-end resistor (connected to ground) will be in parallel with 43.2K.
You can simplify the calculations by using just the upper resistor of the voltage divider. This is really just a series resistor that is in series with the 33.2K of the internal voltage divider.
The maximum voltage at the input of the analog converter is 2.5V (VGX user's manual for electrical specifications).
In this case, Rs being your resistor and Vi your maximum input voltage:
R1 = 10K (internal)
R2 = Rs + 33.2K
Vo = Vi * [R1/(R1+R2)]
Which breaks down to:
Rs = (Vi/2.5)*10K - 43.2K
Round the result up to next standard resistor value.
If this still doesn't solve the problem, please send us more information about the resistance values you used and what results you observed. You can also email us to support.us@eurotech.com. |
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innomatix
12 Posts |
Posted - 22 Jun 2009 : 09:51:10
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| Thanks for the update. We have a spreadsheet available showing our tests and plots of the resulting output. Can we send it to you directly? Thanks. |
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cbadr
154 Posts |
Posted - 22 Jun 2009 : 09:58:49
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| Sure, either to my email or to the Support email address. We look forward to working with you on this issue and we'll post an update for other interested users once resolved. |
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cbadr
154 Posts |
Posted - 23 Jul 2009 : 14:59:07
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| The customer (innomatix) tracked down the issue to be related to an old revision of the VGX board (rev A). Later revisions of the board did not exhibit any problem on the voltage divider. |
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