Section 5: Basic Digital Multimeter (DMM) Operation Series 3700 System Switch/Multimeter User's Manual5-14 Document Number: 3700S-900-01 Rev. A / August 2007Low level considerationsFor sensitive measurements, external considerations beyond the Series 3700 affect theaccuracy. Effects not noticeable when working with higher voltages are significant in microvoltsignals. The Series 3700 reads only the signal received at its input; therefore, it is important thatthis signal be properly transmitted from the source. The following paragraphs indicate factorsthat affect accuracy, including stray signal pick-up and thermal offsets.ShieldingAC voltages that are extremely large compared with the DC signal to be measured may producean erroneous output. Therefore, to minimize AC interference, the circuit should be shielded withthe shield connected to the Series 3700 input low (particularly for low level sources). Impropershielding can cause the Series 3700 to behave in one or more of the following ways: Unexpected offset voltages. Inconsistent readings between ranges. Sudden shifts in reading.To minimize pick-up, keep the voltage source and the Series 3700 away from strong ACmagnetic sources. The voltage induced due to magnetic flux is proportional to the area of theloop formed by the input leads. Therefore, minimize the loop area of the input leads and connecteach signal at only one point.Thermal EMFsThermal EMFs (thermoelectric potentials) are generated by temperature differences betweenthe junctions of dissimilar metals. These can be large compared to the signal that the Series3700 can measure. Thermal EMFs can cause the following conditions: Instability or zero offset is much higher than expected. The reading is sensitive to (and responds to) temperature changes. This effect can bedemonstrated by touching the circuit, by placing a heat source near the circuit, or by aregular pattern of instability (corresponding to changes in sunlight or the activation ofheating and air conditioning systems).To minimize the drift caused by thermal EMFs, use copper leads to connect the circuit to theSeries 3700.A clean, oxidized-free, copper conductor such as #10 bus wire is ideal for this application. Forswitching modules, use #20 AWG copper wire to make connections. The leads to the Series3700 may be shielded or unshielded, as necessary.Widely varying temperatures within the circuit can also create thermal EMFs. Therefore,maintain constant temperatures to minimize these thermal EMFs. A shielded enclosure aroundthe circuit under test also helps by minimizing air currents.The REL control can be used to null out constant offset voltages.
