MCP960X datasheet(45/57 Pages) MICROCHIP

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DS20005426G-page 45

MCP960X/L0X/RL0X

6.3.3

OPEN AND SHORT DETECTION

CIRCUITS

For the MCP9600/L00/RL00, external circuits can be

added to detect the thermocouple status as open

(physically disconnected) or as short (thermocouple

wire in contact with the system ground or VDD). If a

passive circuit is added to the input stage, then the

circuit loading effect to the MCP9600/L00/RL00 ADC

inputs must be considered. System calibration is also

required to ensure proper accuracy. In addition,

external loads can degrade the device performance,

such as input offset, gain and Integral Nonlinearity

(INL) errors. If a low-impedance active circuit is added,

then both offset and gain errors must be calibrated.

The MCP9601/L01/RL01 has a Thermocouple open

and short-circuit detection mechanism, which is imple-

mented using a Sense-Input pin (VSENSE), as shown in

Figure 1-1. The VSENSE pin and RA and RB resistors

must be connected as indicated in Figure 1-1. For

proper operation, the resistor values must also be

within the specified tolerance range. When open-circuit

or short-circuit conditions are detected, the OC Alert

and SC Alert Active-High Push-Pull outputs are

asserted, respectively.

6.3.3.1

Open-Circuit Detection Technique

For MCP9600/L00/RL00 open-circuit detection, the

Input Range bit (bit 4) of the STATUS register

(Register 5-6), can be used to detect open-circuit

conditions. This would require a few external resistors,

as shown in Figure 6-6. The passive circuit does not

affect the MCP9600/L00/RL00 accuracy. When the

thermocouple is connected, the input Common-mode

voltage is 50% of VDD. When the thermocouple is

disconnected, the voltage at the VIN+ input is 66% of

VDD and the voltage at the VIN- input is pulled low to

VSS. This change forces the Input Range bit to be set.

The

MCP9601/L01/RL01

open-circuit

detection

mechanism operates similarly (see Figure 6-7), and

the detection thresholds are specified as VSiOC and

VSiNOR (see DC Characteristics).

The host controller can momentarily poll the Status bit

to detect the Open-Circuit condition. For the

MCP9601/L01/RL01, the OC Alert pin can are used as

hardware indicator.

FIGURE 6-6:

Adding Open-Circuit

Detection Resistors (MCP9600/L00/RL00).

FIGURE 6-7:

Adding Open-Circuit

Detection Resistors (MCP9601/L01/RL01).

Del Sig

VIN+

VIN-

MCP9600/L00/RL00

Thermocouple

VDD

Del Sig

VIN+

VIN-

Thermocouple

VDD

VSENSE

MCP9601/L01/RL01

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