MCP9600 datasheet(39/50 Pages) MICROCHIP

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 2015-2016 Microchip Technology Inc.

DS20005426B-page 39

MCP9600

6.3

Device Features

6.3.1

I2C ADDRESSING

The MCP9600 supports up to eight devices on the I2C

bus. Applications such as large thermal management

racks with several thermocouple sensor interfaces are

able to monitor various temperature zones with minimal

pin-count microcontrollers. This reduces the total solu-

tion cost, while providing a highly accurate thermal

management solution using the MCP9600.

FIGURE 6-4:

I2C Address Selection

Implementation.

6.3.2

INPUT IMPEDANCE

The MCP9600 uses a switched-capacitor amplifier

input stage to gain the input signal to a maximum

resolution of 2 µV/LSb at 18-bit ADC setting. An

internal input capacitor is used for charge storage. The

differential input impedance ZIN_DF is dominated by the

sampling

capacitor

and

the

switched-capacitor

amplifier sampling frequency. During sampling period,

the charging and discharging of the sampling capacitor

creates dynamic input currents at the input pins.

Adding a 10-100 nF capacitor between the inputs can

improve stability.

Since the sampling capacitor is only switching to the

input pins during a conversion process, the input

impedance is only valid during conversion periods.

During low-power or Shutdown mode, the input ampli-

fier stage is disabled, therefore the input impedance is

ZIN_CM, which is due to the leakage current from ESD

protection diodes, as shown in Figure 6-5.

FIGURE 6-5:

Thermocouple Input Stage.

6.3.3

OPEN AND SHORT DETECTION

CIRCUITS

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 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.

6.3.3.1

Open-Circuit Detection Technique

For open circuit detection, the Input Range Flag 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 accuracy

(The recommended value for RB set to 10 k. When

the

Thermocouple

connected,

the

input

common-mode voltage is 0.5*VDD. And when the

Thermocouple is disconnected, the voltage at VIN+

MCP9600

PIC®

Alert

GND

Types K, J, T,

N, E, B, S, R

VDD

Alert

GND

Types K, J, T,

N, E, B, S, R

VDD

MCP9600

R7A R7B

R2A R2B

eight

MCP9600 on

I2C bus

TABLE 6-2:

RECOMMENDED

RESISTOR VALUES FOR

I2C ADDRESSING

Device #

Command

Byte

Values

RXA (k)RXB (k)

1100 000X

ADDR pin tied to GND

1100 001X

R2A = 10

R2B = 2.2

1100 010X

R3A = 10

R3B = 4.3

1100 011X

R4A = 10

R4B = 7.5

1100 100X

R5A = 10

R5B = 13

1100 101X

R6A = 10

R6B = 22

1100 110X

R7A = 10

R7B = 43

1100 111X

ADDR pin tied to VDD

Note:

Standard 5% tolerance resistors are used in

the table, however, 1% tolerance resistors

provide better ratio matching.

ADDR

VIN+

VIN-

VIN+

VIN-

microcontroller

Unit 2/8

Unit 7/8

RSS

VIN+,VIN-

Sampling

Switch

CSAMPLE

(3.2 pF)

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