Technical Notes for the ICL 7106 LCD A/D Converter

This section presents technical material regarding the 7106-based DPM (digital panel meter).The “standard” DPM circuitry is presented first.The second part addresses the modification needed to shift the readout appropriately for O2 measurements.This is followed by suggestions for modifying any 7106-based DPM to function properly in a general O2 Meter role.

Technical Discussion of the ICL7106 DPM Circuit Operation

The ICL7160 integrated circuit contains a dual-slope analog-to-digital (A/D) converter coupled to a Liquid Crystal Display (LCD) driver.The A/D circuit is of primary interest here.The dual-slope A/D conversion process basically integrates the input voltage for a fixed time, then measures the time to drive this value back to zero by integrating a negative known reference voltage, Vref.For the 7106, the first integration period is 1000 counts of the internal clock, so the displayed output pulse count (clock time) is:

Count = 1000*(Vin/Vref)[i.e., if the input Vin is twice the value of Vref , then the reading would be 2000.]

Now, Vreffor almost all DPMs using the 7106 IC is derived from a simple voltage divider across an internally supplied zener diode voltage of around 2.9-3.0 volts between V+ and COMMON (pin 32), also usually connected to pin 35, the Ref LO input.) [Note: although the datasheets for the 7106 indicate that the ref supply is 2.8 volts below V+, the specs also state the mean voltage to be 3.0!I’ve measured an average of 2.96 volts for this reference.]

For “normal” voltmeter applications, the DPM is scaled to read 2000 counts for an input of 200 mV.This calibration requires Vrefto be set to 100 mV.The PM128A uses the following original divider circuit to develop 100 mV for Vref at the REF HI input pin 36:

 

With this arrangement and V+ at 3 volts positive relative to COMMON, the Vref can be varied from 88 mV to 106 mV, by adjusting the 200W pot.

The DMD3500B DPM, available from Circuit Test Electronics, in Burnaby, BC (Canada), uses a different but equivalent circuit (almost identical to the circuit suggested in the ICL7106 application note), as shown below:


With this alternate arrangement and V+ at 3 volts positive relative to COMMON, the Vref can be varied from 89 mV to 127 mV by adjusting the 315 W pot.

Modifying the ICL7106-based DPM for O2 Meter Use

In the following calculations, we consider an input of 10 mV to the DPM when the O2 sensor is exposed to air at 1 ATM.This is derived from the nominal 10.5 mV sensor output in air, scaled to ~ 10 mV by the panel adjustment.
Although the “standard” ICL7106-based DPM has a full-scale reading of 200 mV (a count of 2000), we want it to read 209 (20.9 with a properly placed decimal) with an input of ~10 mV; therefore we need a Vrefvalue of approximately

Vref= 1000* Vin / 209= 1000* (10 mV)/209 = ~ 47.85 mV

To adjust the conversion factor for our purposes, let’s modify the process used to develop the DPM Vrefto produce ~ 48 mV.

For the PM 128(A): Note that if we replace R2, the 30 kW resistor, with a fixed 62 kW resistor, the REF HI output can then be varied from ~43.3 mV to ~52.7 mV, so our desired value of 48 mV is almost exactly in the center of the R4-adjustable range.The calculations involved proceed as follows:

Determine the exact value of the R2 replacement resistor by assuming the pot (R4) is centered; i.e.,

47.85 mV = 3000mV * 1009/ ( 1109 + Rx ),

where Rxis the “perfect” resistor to replace R2.This is obviously
 
 

Rx= [1009 * ( 3000 / 47.85 )] – 1109 = 62,151 W,

and the standard-valued 62 kW resistor works out quite satisfactorily.

For the DMD-3500B: The calculation process is the same, using the values of divider resistance used in the DMD-3500B; i.e.,

47.85 mV = 3000mV * 908 / ( 1065 + Rx ), or

Rx= [ 908 * ( 3000 / 47.85 )] – 1065 = 55,863 W,

where 56 kW is the nearest “standard” resistor value, and quite satisfactory. Substituting a 56 kW resistor for R4 gives a Vref range of 39.4 mV to 56 mV, which is a nice span around the desired ~48 mV.

Modifying Almost Any ICL7106-based DPM for O2 Meter Use

Study the above circuit description and analyses until you understand it well, or find someone who can.Assuming that the “new” ICL7106-based DPM is set up to measure 200 mV in a full scale reading (the usual specification), the first task is to locate the voltage divider components used to produce the Vref value of ~ 100 mV.If the DPM has a DIP (dual inline package) ICL7106, locate REF HI input, pin 36, and find where it is connected – it will be to one end of at least one resistor on the circuit board.Then find the other resistors in the divider circuit by tracing from this point.It is highly likely that the divider circuit will be very similar to one of the two drawn above. The largest resistance element will be the one to replace, and the desired value may be calculated as was done in the two examples above.[Another approach is to measure the values of resistors on the circuit board, and if you find one in the 24 kW -30 kW range, that’s probably it!It’s unlikely the circuit designer varied far from the 24 kW value suggested in the data sheet.]