Have a question about ADCs or DACs? Ask our Expert, Nick Gray

Hi the Rpt equation has been very helpfull to me, but isnt voltage at the mid point = (Rpt/(Rpt + Rr))*Vr rather than (Vr/(Rpt + Rr))*Rpt.

@ Neal,

What's the difference -

(Rpt/(Rpt + Rr))*Vr =  (Vr/(Rpt + Rr))*Rpt

(Rpt/(Rpt + Rr))*Vr = (1/(Rpt + Rr))*Vr*Rpt =  (Vr/(Rpt + Rr))*Vr*Rpt

MK

Hi

I have a project that reads 4 air pressure sensors, performs some calculations and displays the values on 4 seven digit displays.

To complete the project, I want to include a 6 position rotary selector so I can select different algorithms that the code can apply to the Analogue values coming from the sensors.

The way I thought of doing this is to connect 5v to the Selector with different resistors on each of the 6 output pins. Then the Arduino would check the voltage to determine which position the selector is at.

Problem is that I have used up all 6 analogue inputs.

I guess I could use one of the free digital pins and use an analogue to digital converter but I don’t have a lot of space to be adding more boards. Is there some other way I can do this?

Most of the digital pins are free.

Thanks in advance

Sam

I will highly grateful if you can provide me with the correct signals for DAC8541 because I tried many times to connect 0000H, FFFFH to its binary output hoping to obtain logic 0 and logic 1 respectively but nothing appear at the output except 0. I connected the control signals as follows:

                                                                                      PWRDWN0=0

                                                                                      PWRDWN1=0

                                                                                      -RST to step voltage

                                                                                       Resetselect=0

                                                                                        VRFH=5V

                                                                                         VRFL=0V

                                                                                      VDD=AVDD=5V

                                                                                       DGND=AGND=0V

                                                                                       -CS=-WR=0V   (write case)

I use Multisim-12 in my simulation. I appreciate your help.I teached this to my students to enable them going through practical projects in the Lab. Thank for your help.

e-mail:    mohamedrahman2008@yahoo.com

          With my best wishes

I am sorry that I did not see your question sooner, but I believe I have an answer for you.

From your description, I notice that you have -RST “to step voltage”, which I take to mean a rising signal, while Resetselect is held at a logic low. Table 1 on page 14 of the data sheet indicates that this causes the output to be reset to min scale (meaning zero when VRFL is zero). I suggest you try this:

With the desired data at the data input pins, and while holding R/-W low and holding PD0 and PD1 low, bring -CS from a logic high to a logic low, holding it low for at least 20ns, then back to a logic high. Then change R/-W to a logic high and then again toggle -CS from a logic high to a logic low, holding it high for at least 40ns, then back to a logic high. Be sure to observe all of the minimum timing requirements of Figure 1 of data sheet page 5. Also, be sure that the V_OUT pin is connected to the V_OUTSense pin. This connection should be made right at the load for best accuracy.

Please let me know how it works out.

Nick Gray

Hi, Sam –

I am sorry that I did not see your question sooner, but I cave a couple of solutions for you. First, realize that it is not necessary to use a board level ADC as many ADC chips are easy to use. One example is the SPI output ADC081S021. I can suggest a complete circuit, if you need it.

Another solution is to use a string of 12 resistors with 6 voltage comparators. I know of only one device with six voltage comparators in it and that is the LMV7231 from TI. However, there are a few quad voltage comparators available and you can use two of those. The arm of your selector switch would be connected to the non-inverting input of all voltage comparators. The series resistor string would have 13 nodes, numbered 0 through 12, with node 0 at ground and node 12 at perhaps 5 Volts. The even nodes 2 through 10 plus ground are each connected to separate pins of the rotary switch and the odd nodes are each connected to the inverting node of separate voltage comparators. As higher voltages are selected, more and more comparator outputs go high.

Other topologies are possible, but I think these two provide the fewest components.

Nick Gray

I do not know if you ever received an answer, but I do not see one. The term you show as

    (Vr/(Rpt + Rr))*Vr*Rpt

is in error. It should be

     (Vr/(Rpt + Rr))*Rpt

That is, the "Vr" term near the end should not be there.

Nick Gray

Quite right - my only excuse is that trying to wrestle with the E14 editor and do algebra at the same time was too much for me

MK

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