Discussion in "Project Help" started by    TB    Aug 9, 2008.
Sat Aug 09 2008, 11:50 AM
#1
I'm using a Dallas 8051 (DS89C450) for my microcontroller project and it's working fine with serial and a LCD, but now I want to add a MAX192 seriel ADC to it. I have a rough idea of how to write the software for it, but I'm not sure on how to connect it to my board. (I'm a programmer, not an electrical engineer )

At the moment I use Port1 for the LCD and three pins on P3 to control the LCD. Port 0 and Port 2 are not used at all, and 5 pins on port3 are not used. I guess I can use the free pins on port3 to the ADC? If I understand the datasheet of the ADC correct I need the following pins on the microcontroller:

ChipSelect (CS)
SCLK
DIN
DOUT

But what about VRef ? (Can I connect this to VDD?)

To put it shortly (isn't this too late now? ) how do I have to connect it? Where do I need capacitors, and where do I need resistors?
Tags free pinsmicrocontroller projectelectrical engineerrough ideaadc
Sat Aug 09 2008, 03:59 PM
#2
well ..
it is generally preferred not to connect VREF to VDD.

mostly in case of sensitive ADC application u either use two separate power supplies... for Analog and digital sections or simply isolate digital ground from analog ground.

but if it is just an experiment... just add a variable resistor/pot (small value) between VDD and Vref.
u are also supposed to add a zero ohm impedance between your analog signal and the ADC analog i/p pin.(use any resistance around 50E to gain the same effect.)


Tags variable resistorohm impedancedigital sectionsanalog signaladc
Sat Aug 09 2008, 04:07 PM
#3
Have a look at this thread .. http://www.8051projects.net/forum-t8042-last.html

For Vref you can use the typical operating ckt in pg 24.It allows for a single-ended input of 0-4.096 V with a 4mV step size.
Tags cktpg
Sat Aug 09 2008, 05:08 PM
#4


Have a look at this thread .. http://www.8051projects.net/forum-t8042-last.html

sashijoseph




Thank you for the link, but it doesn't say anything about the schematics :/ I think I have a good understand of how to interface the max192 - it's the schematics I'm not sure about. Can I just connect CS, Din and Dout to three pins on my microcontroller? (eg. P3) or do I have to use some resistors, capacitors and so on?


For Vref you can use the typical operating ckt in pg 24.It allows for a single-ended input of 0-4.096 V with a 4mV step size.

sashijoseph



Hmm, I need to monitor values between 0 and +5.0V - so I have to make a divider to bring the voltage down? (eg. a 4/5 divider)
Tags cktdoutdividermicrocontrollerschematics
Sat Aug 09 2008, 07:56 PM
#5
hi TB,
i think it is sufficient to connect the control pins directly to the uC.
As for the reference i would agree with shashi and shyam that u go for 4.096 internal reference ( looks fine as it has a good temperature stability too(30ppm/deg.)).So, yes , u will have to drop ur input voltage in the ratio 4/5.

using a reference close to the supply voltage of the ADC may create some non-linearity in the conversion at the full scale level ( do not know about this ADC as i have not used it as yet, but have observed this in many other ADCs i have used. )
Tags internal referencetemperature stabilityadcsshashishyam
Sat Aug 09 2008, 09:51 PM
#6
pdi: Thank you for your reply

What about the VDD, Ground and so on - do I need any capacitors or resistors there?
Tags resistorscapacitorsreply
Sun Aug 10 2008, 04:35 AM
#7
Fig 18 in the datasheet shows just that.
In general you can directly connect VDD,unless the supply is very noisy,in which case use a 10ohms resistor(which forms a low pass in conjunction with the bypass caps) as specified in the datasheet.
Usually a 0.1uF bypass cap from VDD to Agnd pin(as close to the pins as possible) suffices.
Tags datasheetresistorpinsconjunction

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