LTC2984 datasheet(65/78 Pages) AD

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LTC2984

2984fb

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If excessive RC time constants are present in external

sensor circuits (large bypass capacitors used for thermis-

tors or RTDs) it is possible to increase the settling time

between current source excitation and MUX switching.

The extra delay is determined by the value written into

the MUX configuration delay register (memory location

0x0FF). The value written into this memory location is

multiplied by 100µs; therefore, the maximum extra MUX

delay is 25.5ms (i.e. 0x0FF = 255 • 100µs).

GLOBAL CONFIGURATION REGISTER

The LTC2984 includes a global configuration register

(memory location 0x0F0, see Figure 40). This register is

used to set the notch frequency of the digital filter and

temperatureresultsformat(°Cor°F).Thedefaultsettingis

simultaneous 50/60Hz rejection (75dB rejection with 1ms

MUX delay). If higher 60Hz rejection is required (120dB

rejection),write0x01intomemorylocation0x0F0;ifhigher

50Hz rejection is required (120dB rejection) write 0x02

into memory location 0x0F0.

SUPPLEMENTAL INFORMATION

The default temperature units reported by the LTC2984

are °C. The reported temperature can also be output in °F

by setting bit 3 of memory location 0x0F0 to 1. All other

global configuration bits should be set to 0.

REFERENCE CONSIDERATIONS

The mechanical stress of soldering the LTC2984 to a PC

board can cause the output voltage reference to shift and

temperature coefficient to change. These two changes are

not correlated. For example, the voltage may shift but the

temperature coefficient may not. To reduce the effects of

stress-related shifts, mount the reference near the short

edge of the PC board or in a corner.

Figure 40. Global Configuration Register

2984 F40

0 = °C

1 = °F

00 50/60Hz REJECTION

01 60Hz REJECTION

10 50Hz REJECTION

11 RESERVED

MEMORY LOCATION 0x0F0

0 0 0 0 0

Figure 41. Custom Thermocouple Example (mV vs Kelvin)

2984 F41

TEMPERATURE (K)

(0mV, 0K)

NOTE:

P0 SHOULD BE THE

EXTRAPOLATION

POINT TO 0K

VOLTAGE (mV)

VOLTAGE < p1

SOFT FAULT

CONDITION

VOLTAGE > p9

SOFT FAULT

CONDITION

(0mV, 273.15K)

CUSTOM THERMOCOUPLES

In addition to digitizing standard thermocouples, the

LTC2984canalsodigitizeuser-programmable,customther-

mocouples (thermocouple type=0b01001, see Table 12).

Custom sensor data (minimum of three, maximum of 64

pairs) reside sequentially in memory and are arranged in

blocks of six bytes of monotonically increasing tabular

data as mV vs temperature (see Table 71).

Table 71. Custom Thermocouple Tabular Data Format

ADDRESS

BYTE 0 BYTE 1 BYTE 2 BYTE 3 BYTE 4 BYTE 5

0x250 + 6* Start Address

Table Entry #1 (mV) Table Entry #1 (Kelvin)

0x250 + 6* Start Address + 6 Table Entry #2 (mV) Table Entry #2 (Kelvin)

0x250 + 6* Start Address + 12 Table Entry #3 (mV) Table Entry #3 (Kelvin)

•

Max Address = 0x3CA

Table Entry #64 (mV) Table Entry #64 (Kelvin)

Custom Thermocouple Example

In this example, a simplified thermocouple curve is

implemented (see Figure 41). Points P1 to P9 represent

the normal operating range of the custom thermocouple.

Voltage readings above point P9 result in a soft fault and

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