MCP6V06T datasheet(30/44 Pages) MICROCHIP

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MCP6V06/7/8

DS22093B-page 30

4.4.3

THERMOCOUPLE SENSOR

Figure 4-18 shows a simplified diagram of an amplifier

and temperature sensor used in a thermocouple

application. The type K thermocouple senses the

temperature at the hot junction (THJ), and produces a

voltage at V1 proportional to THJ (in °C). The amplifier’s

gain is is set so that V4/THJ is 10 mV/°C. V3 represents

the output of a temperature sensor, which produces a

voltage proportional to the temperature (in °C) at the

cold junction (TCJ), and with a 0.50V offset. V2 is set so

that V4 is 0.50V when THJ –TCJ is 0°C.

EQUATION 4-5:

FIGURE 4-18:

Thermocouple Sensor;

Simplified Circuit.

Figure 4-19 shows a more complete implementation of

this circuit. The dashed red arrow indicates a thermally

conductive connection between the thermocouple and

the MCP9700A; it needs to be very short and have low

thermal resistance.

FIGURE 4-19:

Thermocouple Sensor.

The MCP9700A senses the temperature at its physical

location. It needs to be at the same temperature as the

cold junction (TCJ), and produces V3 (Figure 4-16).

The MCP1541 produces a 4.10V output, assuming

VDD is at 5.0V. This voltage, tied to a resistor ladder of

4.100(RTH) and 1.3224(RTH), would produce a Theve-

nin

equivalent

1.00V

and

250(RTH). The

1.3224(RTH) resistor is combined in parallel with the

top right RTH resistor (in Figure 4-18), producing the

0.5696(RTH) resistor.

V4 should be converted to digital, then corrected for the

thermocouple’s non-linearity. The ADC can use the

MCP1541 as its voltage reference. Alternately, an

absolute reference inside a PICmicro® can be used

instead of the MCP1541.

4.4.4

OFFSET VOLTAGE CORRECTION

Figure 4-20 shows a MCP6V06 correcting the input

offset voltage of another op amp. R2 and C2 integrate

the offset error seen at the other op amp’s input; the

integration needs to be slow enough to be stable (with

the feedback provided by R1 and R3).

FIGURE 4-20:

Offset Correction.

4.4.5

PRECISION COMPARATOR

Use high gain before a comparator to improve the

latter’s performance. Do not use MCP6V06/7/8 as a

comparator by itself; the VOS correction circuitry does

not operate properly without a feedback loop.

FIGURE 4-21:

Precision Comparator.

V1 ≈ THJ(40 µV/°C)

V2 = (1.00V)

V3 =TCJ(10 mV/°C) + (0.50V)

V4 =250V1 +(V2 –V3)

≈ (10 mV/°C) (THJ –TCJ) + (0.50V)

(RTH)/250

(RTH)

(RTH)/250

(RTH)

MCP6V06

Type K

40 µV/°C

(RTH)

(hot junction

(cold junction

Thermocouple

at THJ)

at TCJ)

RTH = Thevenin Equivalent Resistance

(RTH)/250

0.5696(RTH)

(RTH)/250

(RTH)

MCP6V06

Type K

(RTH)

4.100(RTH)

MCP9700A

VDD

MCP1541

VDD

RTH = Thevenin Equivalent Resistance (e.g., 10 kΩ)

MCP6V06

MCP6XXX

VDD/2

VIN

VOUT

MCP6V06

VIN

VDD/2

MCP6541

VOUT

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