Product Catalog

M.C. Miller Selectable High Impedance Adapter

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  • Improves accuracy of P/S readings.
  • Compensates for high resistance contacts between soil and electrode.
  • Detects high contact resistance at the test lead / structure connection.
  • Operates with any voltmeter.
  • Switch-selectable input impedance of 20, 50, 100, 200 MOhm, and 1GOhm / Open.
  • Superior filtering of AC noise.
  • Watertight enclosure - small and convenient.
  • Has a bypassswitch which allows voltages to be measured directly by bypassing the high impedance buffer, including voltages larger than 4V

When structure-to-soil readings are taken across highly resistive materials such as dry soil, sand, rock, and concrete, readings can appear lower than their actual values. Relying on such low readings can lead to over compensation of the rectifiers' output, overprotection of the structure, and wasted resources.

No longer is it necessary to rely on the fixed 10 MOhm input impedance of standard voltmeters and data loggers for dependable readings. The solution to this common difficulty is the Selectable High Impedance Adapter. The adapter enables any standard impedance voltmeter or data logger to vary its input impedance, thereby eliminating contact resistance errors from readings.

General Information:

Due to the limitations of standard voltmeters, an error can be realized while performing readings with high impedance sources (e.g., structure-to-soil readings at the point where a copper / copper sulfate reference electrode makes contact with the earth). This error is a result of a current loop created by the voltmeter, which causes a drop across the source resistance. The Selectable High Impedance Adapter varies this current loop, allowing the user to determine when the error has been made insignificant. However, with higher impedance there is an increase in noise pickup. Therefore, when two adjacent ranges give the same readings, select the lower of the two ranges to limit pickup while maintaining the most precise reading.


The Selectable High Impedance Adapter is a multipurpose device used to vary the input impedance of data loggers and voltmeters, as well as a filter to remove AC noise.

The high impedance adapter has excellent AC noise rejection capabilities that improve as the frequency of the noise increases. Noise at a frequency of 50 Hz and 6.36 Vrms (9 V peak) produces only 1 count of error (+1.5 mV) at the output, while at 60 Hz, it takes 19.1 VRMS (27 Vpeak of noise) to cause 1 count of error.

The selectable input impedance feature permits detection of high resistance in the external circuit and the elimination of resulting errors in virtually all cases. This feature is implemented by first taking the reading with the High Impedance Adapter in the 20 MOhm position and switching to the next higher impedance while watching the output until the reading is constant. Next, turn the switch back one position to decrease the impedance, which will reduce any noise pickup and will result in the most precise reading.

In addition to giving precise readings, the High Impedance Adapter can also withstand a constant DC input of 700 volts as well as 1.5 kV voltage spikes.

The OFF position of the adapter functions as a means of testing its internal battery from a digital multimeter. To test the battery, turn the switch to the OFF / BAT TEST position and connect the High Impedance Adapter's output to a digital multimeter and set it to the 200 mV range. A reading below 80 mV requires that the battery be changed.

DC Response and Accuracy:

It takes the High Impedance Adapter less than 200ms to reach an output voltage of 2VDC +1mV with a 2VDC input. Maximum operational DC input: +4V.

Input Impedance: Switch selectable (20, 50, 100, 200MΩ, 1GΩ). Definitions:

Rejection is defined as the 60 Hz (50 Hz) RMS voltage required to cause the corresponding counts of ripple.
Settling Time is defined as the time for the output to be within the corresponding counts of ripple for each rejection.
Counts of Ripple refers to the amount of amplitude variation due to AC noise.

Selectable Bypass Polarity Switch:

1. In the ON position, the HIB will work in normal working capabilities.
2. In the OFF position, The HIB is completely turned off.
3. In the BYPASS position the HIB is being bypassed thru that is higher then 4V. Switch capacity is at 5A.


9V alkaline or 9V lithium recommended.

Battery Life:

Alkaline - greater than 60 hours of continuous operation (worst case).
Lithium - greater than 210 hours of continuous operation (worst case).

Minimum battery voltage:

8.0 Volts

Operating temperature:

-40oF to 185oF (-40oC to +85oC).


Overall: 3-1 /4" x 3-9/16" x 3" (8.3 x 9.0 x 7.6 cm)
Box: 3-1/4"x 3-3/16 x 2-3/16" box (8.3 x 8.1 x 5.6 cm)


9.3 oz. (264 g)


Mounting plate for attaching the Selectable High Impedance Adapter to the M. C. Miller Co., Inc. SinCorder.