Datasheet LTC1050 (Analog Devices) - 6

LTC1050
TEST CIRCUITS Electrical Characteristics Test Circuit DC-10Hz Noise Test Circuit
1M 100k 475k 0.015µF V+ 1k 10Ω 2 – 7 – 158k 316k 475k 6 LTC1050 OUTPUT LTC1050 – 3 + TO X-Y R + 0.015µF 0.015µF LT®1012 4 L RECORDER + 1050 TC01 V – FOR 1Hz NOISE BW, INCREASE ALL THE CAPACITORS BY A FACTOR OF10 1050 TC02
O U U W U APPLICATI S I FOR ATIO ACHIEVING PICOAMPERE/MICROVOLT
V +
PERFORMANCE Picoamperes
8 OUTPUT 7 1 In order to realize the picoampere level of accuracy of the 6 LTC1050, proper care must be exercised. Leakage currents OPTIONAL 2 EXTERNAL 5 in circuitry external to the amplifier can significantly degrade CLOCK 4 3 performance. High quality insulation should be used (e.g., Teflon, Kel-F); cleaning of all insulating surfaces to remove INPUTS fluxes and other residues will probably be necessary— particularly for high temperature performance. Surface V – GUARD 1050 F01 coating may be necessary to provide a moisture barrier in high humidity environments.
Figure 1
Board leakage can be minimized by encircling the input connections with a guard ring operated at a potential close EMF generation. Junctions of copper wire from different to that of the inputs: in inverting configurations the guard manufacturers can generate thermal EMFs of 200nV/°C— ring should be tied to ground; in noninverting connections 4 times the maximum drift specification of the LTC1050. to the inverting input (see Figure 1). Guarding both sides The copper/kovar junction, formed when wire or printed of the printed circuit board is required. Bulk leakage reduc- circuit traces contact a package lead, has a thermal EMF of tion depends on the guard ring width. approximately 35µV/°C—700 times the maximum drift specification of the LTC1050.
Microvolts
Minimizing thermal EMF-induced errors is possible if ju- Thermocouple effect must be considered if the LTC1050’s dicious attention is given to circuit board layout and ultralow drift is to be fully utilized. Any connection of dis- component selection. It is good practice to minimize the similar metals forms a thermoelectric junction producing number of junctions in the amplifier’s input signal path. an electric potential which varies with temperature (Seebeck Avoid connectors, sockets, switches and relays where effect). As temperature sensors, thermocouples exploit this possible. In instances where this is not possible, attempt phenomenon to produce useful information. In low drift to balance the number and type of junctions so that differ- amplifier circuits the effect is a primary source of error. ential cancellation occurs. Doing this may involve Connectors, switches, relay contacts, sockets, resistors, deliberately introducing junctions to offset unavoidable solder and even copper wire are all candidates for thermal junctions. 1050fb 6