Electronic Design

  
Reprints     Printer-Friendly    Email this Article    RSS        Font Size     What's This?


[Ideas For Design]
Use Excel To Calculate A-D Level-Shifter Resistor Values

Aubrey Kagan  |   ED Online ID #12123  |   March 16, 2006


Many times, the need arises to interface single-supply analog-to-digital converters (ADCs) and comparators to real-world signals like 5 V. Of course, it's possible to condition the signal using operational and/or instrumentation amplifiers. But few engineers realize that it's often possible to achieve the level shifting using a resistor network (Fig. 1).

Critics of this technique point out that the resistor network can load the source voltage and cause distortion. We can build this limitation into Microsoft Excel to ensure the resistor values don't overload the input. Another possible concern is that some ADCs won't run properly with a high source impedance, so you would probably have to buffer with a suitable operational amplifier.

To calculate resistor values, we need to use Kirchhoff's Law: The sum of currents into a node is zero. Considering the node at the junction of the three resistors, we can write:

(VIN - VAD)/R1 + (VDD - VAD)/R2 + (VSS - VAD)/R3 = 0

(1)

As an example, let us assume VDD = 5 V, VSS = 0 V, VIN = 5 V, and the ADC input must go from 0 to 2.5 V. For VIN = -5 V, we want VAD = 0 V. So we can substitute the values in Equation 1:

(-5/R1) + (5/R2) = 0

(2)

For VIN = +5 V, we want the input to be 2.5 V, so substituting in Equation 1:

(2.5/R1) + (2.5/R2) - (2.5/R3) = 0

(3)

We have two equations with three unknowns, leaving one degree of freedom. Now we can go ahead and solve this. If we reduce the generalization so that VSS is always 0, we can rearrange Equation 1 as follows:

VAD = [(-R2 × R3 × VIN ) - (R1 × R3 × VDD )]/[(-R1 × R2) - (R2 × R3) - (R1 × R3)]

(4)

Each time we do this though, it's a tedious process. But Excel has a feature aptly called "Solver," which will trivialize the whole exercise once it's set up. To use Solver, you must enable it as follows:

In Windows, go to Control Panel and select the Add/Remove Software option. Select the Microsoft Office entry, and then opt for changing or updating the installation. Find Excel in the list and for the Solver Add-in, select "Run from my computer." Follow the prompts to complete the installation. Now start Excel. Click on Tools, followed by Add-Ins, and enable the Solver Add-In. Follow whatever installation prompts occur (if any).

After loading the worksheet, ADinput.xls, which can be found at www.electronicdesign.com, open it to reveal Figure 2. The input parameters are in cells B5 through B9. Cell B14 contains Equation 1 suitably modified for the minimum condition as follows:

=((VinMin-VadMin)/RE1)+((Vdd-VadMin)/RE2)-(VadMin/RE3)

Similarly cell B15 is modified for the maximum condition:

=((VinMax-VadMax)/RE1)+((Vdd-VadMax)/RE2)-(VadMax/RE3)

Cells B20 and B21 contain the formula for the input current. Notice that it sources when the maximum input voltage is applied and sinks for the minimum.

To invoke Solver, click on Tools | Solver and then make sure the entry parameters are as in Figure 3. Solver will modify cells B10 through B12, trying to keep cells B14 and B15 at zero and cells

B20 and B21 below and above predetermined project parameters. In this case, the current is limited to ±0.1 mA. You can modify this by selecting the constraint and clicking on the Change button. Note that you can work with two or more target cells by constructing all but one of the targets as constraints.

Cell B18 is used to prove that the resistor values do, in fact, generate the correct voltage range (Figure 4). By changing the value in cell B17, you can see how the value of Vad changes. Cell B17 contains Equation 4. It's possible to use Excel to generate the standard resistor values as well, but this isn't described here.

Recommended Reading:

  1. 1. Kagan, Aubrey, Excel by Example: A Microsoft Excel Cookbook for Electronics Engineers, Newnes 2004, ISBN 0750677562
  2. 2. National Semiconductor, Datasheet ADC0801, DS005671, Nov. 1999


Reprints   Printer-Friendly  Email this Article  RSS    Font Size   What's This?


  • A New Design Inflection Point
  • Forecasting Industry Growth For 2009 And Beyond
  • EDA Retools To Exploit Multicore Architectures
  • Design And Verification Move Up In Abstraction
  • EDA Retools To Exploit Multicore Architectures
  • A New Design Inflection Point
  • Design And Verification Move Up In Abstraction
  • Challenges Lurk For 22-nm Physical Implementation
    1) 1-A Switching Regulators Operate With 96% Efficiency To Replace Linear Regulators
    (535 views today)
    2) Battery Pack Improves Li-Ion Management For Electric Vehicles
    (311 views today)
    3) New Power Approaches May Fuel Analog Job Opportunities In Security And Health Applications
    (304 views today)
    4) Build A Smart Battery Charger Using A Single-Transistor Circuit
    (281 views today)
    5) Step-Down Switching Regulator Provides 60-V Input Transient Protection
    (157 views today)
    ALL TOP 20



    Reader Comments

    ADinput.xls can be found at

    http://electronicdesign.com/files/29/12123/12123.zip

    Anonymous -December 07, 2007

    The article says ADinput.xls can be found at www.electronicdesign.com. I couldn't find the link to the Excel file.

    Philipp V. -August 15, 2007

    MANY THANKS FOR YOUR FRUITFUL PRACTICAL IDEAS

    DE SANTOS -March 25, 2006

    well done article Thank You

    Anonymous -March 18, 2006   (Article Rating: )

    POST YOUR COMMENTS HERE
    Name:

    Email:
    Your Comments:

    Enter the text from the image below


    Please refresh the page if you have trouble reading this text.

    Search Electronic Design
         
      
     
    Email Newsletter
    Sponsored By:
    Electronic Design UPDATE provides readers with late-breaking news, opinions from industry experts, and timely technology stories. It's a unique opportunity to get your product message in front of engineers, engineering managers, and corporate managers while they're reading about critical information online.

    Enter Email to Subscribe
      

    Electronic Design Europe Electronic Design China EEPN Power Electronics Auto Electronics Microwaves & RF
    Mobile Dev & Design Schematics Find Power Products Military Electronics EE Events Related Resources