Measurement Computing 6222 - Manual (Page 14)

3-2 Thermocouple Input 897690 6222 User ’s Manual
Circuitry
The following figure represents a circuit for one channel. The two 10M ohm resistors produce input impedance
at the channel’s positive and negative terminals. The gain and offset errors that result from the source
impedance of the thermocouple will be negligible for most applications. Other voltage sources (with higher
source impedance) can introduce more significant error. Refer to the Specifications chapter for more
information about errors that result from source impedance.
Accuracy
This section pertains to various factors that have an effect on temperature measurement error. These are: the
type of thermocouple being used, the temperature being measured, the accuracy of the thermocouple, and the
cold-junction temperature.
Accuracy for Different Thermocouple Types
The following figures show the typical and maximum errors for the different thermocouple types when used
with the 6222 over the full temperature range. The figures also show the maximum error for the thermocouple
types with the 6222 at room temperature (15°C to 35°C). The figures account for gain errors, offset errors,
differential and integral nonlinearity, quantization errors, noise errors, and isothermal errors. The figures do not
account for the accuracy of the thermocouple itself.
Temperature gradients across the 6222 terminals affect the cold-junction temperature accuracy. Refer to the
upcoming Cold-Junction Temperature Measurement Accuracy sub-section for more information about
temperature gradients.
Analog Input Circuit for 1 Channel