LTC2990
12
2990fe
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applicaTions inForMaTion
Device Configuration
The LTC2990 is configured by writing the control register
through the serial interface. Refer to Table 5 for control
register bit definition. The device is capable of many ap
-
plication configurations including voltage, temperature and
current 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
register does not interrupt measurement data updates.
In a 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 Tables 8 and 9,
each with 0.0625 degree-weighted LSBs. The format is
controlled by the control register, Bit 7. All temperature
formats, T
INT
, T
R1
and T
R2
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 cor
-
responding register will be high if the remote sensor diode
differential voltage is above 1.0V
DC
. 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 indicated 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 respec
-
tive 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 register contents
have been accessed since the result was 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):
V
SINGLE-ENDED
= D[14:0] • 305.18µV, if Sign = 0
V
SINGLE-ENDED
= (D[14:0] +1) • –305.18µV, if Sign = 1
V
DIFFERENTIAL
= D[14:0] • 19.42µV, if Sign = 0
V
DIFFERENTIAL
= (D[14:0] +1) • –19.42µV, if Sign = 1
