Input Resistance

All instrumentation modules, which actively gather data, must extract energy from sensors in order to measure information. This "energy extraction" will, of course, create serious errors if allowed to be excessive. We see the significance of this error reflected as "input" specifications of signal conditioning modules. For example, consider a voltage input module, which has an input impedance (Rin) of 1000 ohms and is measuring a 5 volt voltage source with an internal source resistance (Rs) of 4000 ohms. In this example, the module "sees" only a 1 volt signal, a negative 80% error. Ohm's Law voltage divider phenomenon, Vin = Vs / (1+Rs/Rin) models this behavior. If Rin is significantly greater than Rs then the ratio Rs / Rin is << 1 and Vin approximately equals Vs.

Generally, an acceptable error budget has been established in instrumentation projects. Signal conditioning modules should be selected so that errors caused by the Rs / Rin ratio are negligible. The simple expression, Rin >> [(100 / %error - 1) * Rs] can assist in this determination. To illustrate, assume a total error budget of 0.01% with 0.001% allocated to input resistance, this expression shows that Rin must be greater than 99,999 * sensor source resistance, Rs. Given this assumed error budget, the instrument engineer can choose an appropriate combination of sensor and signal conditioning module.

Dataforth has a complete line of signal conditioning modules with high input impedances specified for the module "on", the module "off", and for the module input under "overload". Ask for, and compare, these specifications when selecting competitive signal conditioning products.

For an example of Input Resistance specifications, please take a look at one of Dataforth's typical spec sheets, on page 2 of the following:
Dataforth linearized thermocouple conditioner -- DSCA47 (167Kb pdf).

Remember, our Application Engineers can assist you with signal conditioner selection over the phone or via fax and email. Call us at our manufacturing facility in Tucson at 520-741-1404 (fax 520-741-0762) or Email us at techinfo@dataforth.com.
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