Reference Resistors Ladder Introduction


In this stage, we will install and test the reference resistors and the connectors for the item (R, L, or C) that is to be measured.

The "ladder" of resistors is a set of precision (1%) resistors each of which can be switched into the circuit to form the top half of a voltage divider, the bottom half of which is the component under test.

The switching is performed by 5 of the 6 positions in the 2 pole 6 position switch, S1.

In this stage, you will be measuring resistances whose precise values are important to the accuracy of the meter. If possible, you should use/find/borrow a fairly accurate ohmmeter for this stage. This is clearly not a job for the $1.99 Harbor Freight special!

Theory of Operation

This portion of the circuit builds the variable resistance voltage divider, consisting of a selacted referece resistor (Rm) coupled with the unknown impedance (Z?), divides the input reference AC voltage (Vr) and outputs the ac voltage (Vx) that is dropped across Z?.

Switch S1 allows the user to select a value of Rm appropriate to the component to be measured.

Vr comes from the PC's soundcard at a known, reference frequency and a known, reference voltage value. Vr is input to the voltage divider and also goes to the OpAmp stage where it is buffered by a unity-gain opamp for input to one of the soundcard's two (stereo) channels.

Vx goes to the OpAmp stage where it is buffered in a unity-gain opamp for input to the other (stereo) channel of the PC's soundcard.

(go directly to build notes)

Reference Resistors Ladder Schematic

(Click for Full Schematic)
Reference Resistors Ladderschematic

(above schematic has clickable areas that can be used for navigation)

(go directly to build notes)

Reference Resistors Ladder Bill of Materials

Stage Bill of Materials

(resistor images and color codes courtesy of WIlfried, DL5SWB's R-Color Code program)

P1wire and alligator clips Cable P1 should have 1, short, red wire with a clip at the end.Reference Resistors Ladder
P2wire and alligator clips Cable P2 (gnd) should have 1, short, black wire with a clip at the end.Reference Resistors Ladder
R110 ohm 1/4W 1%br-blk-blk-gld-br br-blk-blk-gld-br1/4W  Reference Resistors Ladder
R2100 ohm 1/4W 1%br-blk-blk-blk-br br-blk-blk-blk-br1/4W  Reference Resistors Ladder
R31 k 1/4W 1%br-blk-blk-br-br br-blk-blk-br-br1/4W  Reference Resistors Ladder
R410 k 1/4W 1%brn-blk-blk-red-brn brn-blk-blk-red-brn1/4W  Reference Resistors Ladder
R5100 k 1/4W 1%brn-blk-blk-orng-brn brn-blk-blk-orng-brn1/4W  Reference Resistors Ladder

Reference Resistors Ladder Summary Build Notes

Reference Resistors Ladder Detailed Build Notes

Top of the Board

Reference Resistors Ladder Top View

Install the reference resistors

Use your most accurate ohmmeter to measure the resistors.
Relying upon color codes, alone, can be risky, depending upon the user's visual acuity and the quality of the light.

Measure the actual Resistance Values


You will use these precise values in the Calibration stage to populate the program's reference resistors table. So, save the values you have measured.

You will also use these values to compare with the test measurements below. The test measurements will vary a fractional amount (due to in-circuit resistances). If there is variation greater than 1%, you should check out and/or touch up all the solder joints you made in this stage.

R110 ohm 1/4W 1%br-blk-blk-gld-br br-blk-blk-gld-br1/4W 
R2100 ohm 1/4W 1%br-blk-blk-blk-br br-blk-blk-blk-br1/4W 
R31 k 1/4W 1%br-blk-blk-br-br br-blk-blk-br-br1/4W 
R410 k 1/4W 1%brn-blk-blk-red-brn brn-blk-blk-red-brn1/4W 
R5100 k 1/4W 1%brn-blk-blk-orng-brn brn-blk-blk-orng-brn1/4W 

Install the measurement leads and clips

It is advisable to install P1 and P2 using very short lengths (so as to avoid adversely affecting the measurement of small low value inductors/capacitors).

P1wire and alligator clips Cable 
P1 should have 1, short, red wire with a clip at the end.
P2wire and alligator clips Cable 
P2 (gnd) should have 1, short, black wire with a clip at the end.

Reference Resistors Ladder Completed Stage

Top of the Board

View of Completed Top

Reference Resistors Ladder Testing

Resistance Switching

Test Setup

This test verifies that the resistors have all been correctly installed and in the correct values and that the switching performs as designed.

unplug the USB if it is connected (i.e., this is a POWER-OFF test)

Clip the leads from P1 and P2 together

Using your ohmmeter, measure the resistance between the lower lead (hole) of R7 and any of the holes for P2, for each of the six positions on S1.

These resistance values will be used in setting the "Rm panel in the software (tells the software what the actual values are in the reference resistance ladder).

Resistance Switching

Test Measurements

TestpointUnitsNominal ValueAuthor'sYours
S1 @ First Position (no resistor switched in)ohms~~_______
S1 @ Second Positionohms1010.7_______
S1 @ Third positionohms100100.4_______
S1 @ Fourth positionohms1 k997_______
S1 @ Fifth positionohms10 k10 k_______
S1 @ Sixth positionohms100 k100.1k_______
To OpASmp Stage To OpASmp Stage To OpASmp Stage