Quick Circuit Check Needed

[rmtucker]

Could anyone just spend a few mins to check the attatched Circuit?
It is just a board for something else i am developing,and i want to cut the pcb on my cnc tommorow.

http://docs-europe.electrocomponents.com/webdocs/0ad9/0900766b80ad92f4.pdf
Link to the Vn920 Datasheet

 

[MartinM57]

Looks OK, but I still wouldn't commit to a PCB, even if it's home made CNC, without breadboarding first....

 

[John West]

have you read pg, 8/20 of the data sheet and decided you didn't need the extra protection circuitry? If so, give it a try.

Make sure your picaxe circuit is well protected with capacitor filtration. It looks like circuit noise might be a concern.

 

[Jaguarjoe]

R2 will be a problem. The max input resistance (or impedance) feeding the ADC should be 2k5 ohms or less.

 

[rmtucker]

If R2 is a problem what is the alternatives if any.
I believe according to the datasheet that R1 and R2 or 10k resistors were just to protect the picaxe?.

 

[MartinM57]

R2 is fine - 10K is the accepted max impedance for an analog input. Picking a PICAXE at random (20X2) the 18F14K22 data sheet says

The maximum recommended impedance for analog sources is 10 kΩ.

 

[rmtucker]

The board will be part of a 28x2 circuit initially then a 40x2 in the finished board.
We seem to have conflicting opinions or is it just because i did not state which picaxe (Sorry).

 

[Jaguarjoe]

The data sheet for the 28X2's parent chip in section 27 near the end says that the maximum input impedance should be 3K or less. 2K5 is pretty close. But irregardless of the value is, if you buffer the signal feeding the Picaxe with an op-amp arranged as a simple voltage follower, you'll be golden.

Note well: Manual 1 says the max resistance should be 20K or less. Totally out in left field.

 

[hippy]

My datasheet for the 28X2 (18F25K22) on page 302 and 312 says 10K. Table 27-21 does say 3K max but that is water-mark stamped as "preliminary".

Always take the datasheet figures over the PICAXE manuals as they may reflect information for earlier PICAXE devices.

 

[John West]

The board will be part of a 28x2 circuit initially then a 40x2 in the finished board.
We seem to have conflicting opinions or is it just because i did not state which picaxe (Sorry).

In case you dig into the PIC data sheets you'll want to know that there has been a recent upgrade in the 28X2. (The 40X2 chips as well, as I recall.) The old 28X2 version is the PIC18F2520. The new one is the PIC18F25K22.

Depending where you purchase yours from, you might still get either version.

 

[hippy]

We recommend references to "28X2" refer to the latest device only, 28X2-5V and 28X2-3V to refer to earlier devices ...

28X2 = 18F25K22, Firmware B.3
28X2-5V = 18F2520, Firmware B.0-B.2
28X2-3V = 18F25K20, Firmware B.A

40X2 are similar with "18F45K22", "18F4520" and "18F45K20" codes.

 

[John West]

All these identifiers, versions and upgrades are certain to keep picaxe life interesting, especially with some of the earlier chip versions still floating around.

I don't at all envy tech support in that regard. Combined with software revisions and documentation revisions there's just too much to know and keep up with. Good luck!

 

[rmtucker]

The picaxe is 28X2 = 18F25K22, Firmware B.3
So the Circuit will be ok?

 

[Dippy]

I think you will find the "prelimary" and "advance information" and other 'cover-my-bottom' remarks can stay on Microchip Data Sheets for months and years.

As your circuit looks 'close enough' I (if it were me) would get some breadboard and (very carefully) simply get on and try it in a 'controlled prototype' manner.

As far as input impedance is concerned , this can be very long subject and can get a bit technical.
Basically, within reason and for PICAXErs, the lower the input impedance the better.
There are a number of ADC-related settings not accessible to PICAXE users which can affect ADC reliabilty - esp. when high speed ADCing is used.

Often you will need to compound the impedances to get close to reality.
Sometimes , as long as the basic valid design specs are covered, careful breadboarding is the easiest and quickest way.

In fact , if you experiment with resistors and pots etc. it can be very educational. Then you'll never need to ask again.... and, believe me, that can save days!

 

[MartinM57]

If you want (re)assurance that the circuit connections (only) are OK, then as long as you are happy that you aren't including the various protection features (JW's post #3) then it looks OK.

If you want (re)assurance that the component values are OK, then you might want to temporarily put a 10K pot in place of the allegedly troublesome resistor and tweak it when there is a constant current through the VN920 and see what the effects are.

 

[Jaguarjoe]

Or, just feed the ADC with a $0.49 op-amp and forget about it. The op-amp will accurately drive any ADC designed after 1967 or so with an input Z of 500 ohms or more.