Selectable High Impedance Adapter
by MC Miller Co.

When structure-to-soil readings are taken across highly resistive materials such as dry soil, sand, rock or concrete, readings can appear lower than their actual values. Relying on such low readings can lead to overcompensation 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 sulphate 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 impedances 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. Selectable High Impedance 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 10 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 to the 200 mV range. A reading below 80 mV requires that the battery be changed. Specifications Frequency Response Frequency Attenuation Counts of Error 50 Hz 60 Hz -74.3 dB -85.6 dB 1 in 5,210 1 in 19,149 AC Rejection at 50 Hz Rejection Counts of Ripple Settling Time 6.36 Vrms 11.3 Vrms 17.7 Vrms 1 (+1.5 mV) 2 (+2.5 mV) 3 (+3.5 mV) <200 ms <200 ms <200 ms AC Rejection at 60 Hz Rejection Counts of Ripple Settling Time 19.1 Vrms 33.2 Vrms 1 (+1.5 mV) 2 (+2.5 mV) <200 ms <200 ms Maximum AC noise before distortion occurs Frequency AC Noise 50 Hz 60 Hz 25.5 Vrms 34.0 Vrms DC Response and Accuracy: It takes the High Impedance Adapter less than 200 ms to reach an output voltage of 2 VDC +1 mV with a 2 VDC input. Maximum operational DC input: +4 Volts.   Input Impedance: Switch selectable (10, 25, 50, 100, 200 MOhm).   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.   Battery: 9 V alkaline or 9 V 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).   Size: 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)   Weight: 9.3 oz. (264 g)   Catalog No.: 5612   Options: Mounting plate for attaching the Selectable High Impedance Adapter to MC Miller Co.'s SinCorder.