LTC2997 datasheet(12/26 Pages) AD

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LTC2990

Rev. F

For more information www.analog.com

APPLICATIONS INFORMATION

Device Configuration

The LTC2990 is configured by writing the control register

through the serial interface. Refer to Table 5 for control reg-

ister bit definition. The device is capable of many applica-

tion configurations including voltage, temperature and cur-

rent measurements. It is possible to configure the device

for single or repeated acquisitions. The device can make

single measurements, or in continuous mode, repeated

acquisitions. When the device is configured for multiple

measurements, the order of the measurements is fixed.

For repeated acquisitions, only an initial trigger is required

after which data registers are continuously refreshed with

new data. As each new data result is ready, the MSB of the

corresponding data register is set, and the corresponding

status register bit is set. These bits are cleared when the

corresponding data register is addressed. The configura-

tion register value at power-up causes the part to measure

only the internal temperature sensor when triggered. The

four input pins V1 through V4 will be in a high impedance

state, until configured otherwise, and a measurement is

triggered. The data registers are double-buffered in order

to ensure upper and lower data bytes do not become out

of sync. Read operations must be terminated in order to

avoid an indefinitely paused wait state. Reading the STATUS

polling system, it is recommended that the STATUS register

be tested for new data, this prevents unnecessary delays

updating the measurement registers.

Data Format

The data registers are broken into 8-bit upper and lower

bytes. Voltage and current conversions are 14-bits. The

upper bits in the MSB registers provide status on the

resulting conversions. These status bits are different for

temperature and voltage conversions:

Temperature: Temperature conversions are reported as

Celsius or Kelvin results described in Table 8 and Table 9,

each with 0.0625 degree-weighted LSBs. The format is

controlled by the control register, Bit 7. All temperature

formats, TINT, TR1 and TR2 are controlled by this bit. The

Temperature MSB result register most significant bit

(Bit 7) is the DATA_VALID bit, which indicates whether

the current register contents have been accessed since

the result was written to the register. This bit will be set

when new data is written to the register, and cleared when

accessed. Bit 6 of the register is a sensor-shorted alarm.

This bit of the corresponding register will be high if the

remote sensor diode differential voltage is below 0.14V.

The LTC2990 internal bias circuitry maintains this voltage

above this level during normal operating conditions. Bit

5 of the register is a sensor open alarm. This bit of the

corresponding register will be high if the remote sensor

diode differential voltage is above 1.0VDC. The LTC2990

internal bias circuitry maintains this voltage below this

level during normal operating conditions. The two sensor

alarms are only valid after a completed conversion indi-

cated by the data_valid bit being high. Bit 4 through Bit 0

of the MSB register are the conversion result bits D[12:8],

in two’s compliment format. Note in Kelvin results, the

result will always be positive. The LSB register contains

temperature result bits D[7:0]. To convert the register

contents to temperature, use the following equation:

T = D[12:0]/16.

See Table 10 for conversion value examples.

Voltage/Current: Voltage results are reported in two

respective registers, an MSB and LSB register. The

Voltage MSB result register most significant bit (Bit 7)

is the data_valid bit, which indicates whether the current

written to the register. This bit will be set when the register

contents are new, and cleared when accessed. Bit 6 of the

MSB register is the sign bit, Bits 5 though 0 represent

bits D[13:8] of the two’s complement conversion result.

The LSB register holds conversion bits D[7:0]. The LSB

value is different for single-ended voltage measurements

V1 through V4, and differential (current measurements)

V1 – V2 and V3 – V4. Single-ended voltages are limited

to positive values in the range 0V to 3.5V. Differential

voltages can have input values in the range of –0.300V

to 0.300V.

Use the following equations to convert the register values

(see Table 10 for examples):

VSINGLE-ENDED = D[14:0] • 305.18µV, if Sign = 0

VSINGLE-ENDED = (D[14:0] +1) • –305.18µV, if Sign = 1

VDIFFERENTIAL = D[14:0] • 19.42µV, if Sign = 0

VDIFFERENTIAL = (D[14:0] +1) • –19.42µV, if Sign = 1

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