Additional Adc Functions For Voltage Measurements; Temperature Measurement System - Analog Devices ADT7460 Manual

ADDITIONAL ADC FUNCTIONS FOR
VOLTAGE MEASUREMENTS
A number of other functions are available on the ADT7460 to
offer the systems designer increased flexibility.
Turn-Off Averaging
For each voltage measurement read from a value register, 16
readings have actually been made internally and the results
averaged before being placed into the value register. If the user
wants to speed up conversion, setting Bit 4 of Configuration
Register 2 (Reg. 0x73) turns averaging off. This effectively gives
a reading 16 times faster (711 µs), but the reading may be noisier.
Bypass Voltage Input Attenuator
Setting Bit 5 of Configuration Register 2 (Reg. 0x73) removes
the attenuation circuitry from the 2.5 V input. This allows the
user to directly connect external sensors or to rescale the analog
voltage measurement inputs for other applications. The input
range of the ADC without the attenuators is 0 V to 2.25 V.
Single-Channel ADC Conversion
Setting Bit 6 of Configuration Register 2 (Reg. 0x73) places the
ADT7460 into single-channel ADC conversion mode. In this
mode, the ADT7460 can be made to read a single voltage
channel only. If the internal ADT7460 clock is used, the selected
input is read every 711 µs. The appropriate ADC channel is
selected by writing to Bits <7:5> of the TACH1 Minimum High
Byte register (Reg. 0x55).
Table 10. Configuration Register 2 (Reg. 0x73)
Bit Description
<4> 1: averaging off
<5> 1: bypass input attenuators
<6> 1: single-channel convert mode
Table 11. TACH1 Minimum High Byte (Reg. 0x55)
Bit Description
<7:5> Selects ADC channel for single-channel convert mode
Value
000
010
TRANSISTOR
Channel Selected
2.5 V
V
CC
I
CPU
THERMDA D+
REMOTE
SENSING
D–
THERMDC
Figure 25. Signal Conditioning for Remote Diode Temperature Sensors

TEMPERATURE MEASUREMENT SYSTEM

Local Temperature Measurement
The ADT7460 contains an on-chip band gap temperature
sensor whose output is digitized by the on-chip 10-bit ADC.
The 8-bit MSB temperature data is stored in the local
temperature register (Address 0x26). As both positive and
negative temperatures can be measured, the temperature data is
stored in twos complement format, as shown in Table 12.
Theoretically, the temperature sensor and ADC can measure
temperatures from −128°C to +127°C with a resolution of
0.25°C. However, this exceeds the operating temperature range
of the device, so local temperature measurements outside this
range are not possible.
Remote Temperature Measurement
The ADT7460 can measure the temperature of two remote
diode sensors or diode-connected transistors connected to
Pins 12 and 13, or Pins 10 and 11.
The forward voltage of a diode or diode-connected transistor
operated at a constant current exhibits a negative temperature
coefficient of about −2 mV/°C. Unfortunately, the absolute
value of V varies from device to device, and individual
BE
calibration is required to null this out, so the technique is
unsuitable for mass production. The technique used in the
ADT7460 is to measure the change in V
operated at two different currents. This is given by
∆ =
V KT
BE
where:
K is Boltzmann's constant.
q is the charge on the carrier.
T is the absolute temperature in Kelvins.
N is the ratio of the two currents.
Figure 25 shows the input signal conditioning used to measure
the output of a remote temperature sensor. This figure shows
the external sensor as a substrate transistor provided for
temperature monitoring on some microprocessors. It could
equally well be a discrete transistor, such as a 2N3904.
V
DD
N × I I
BIAS
LPF
f = 65kHz
BIAS
C
DIODE
Rev. C | Page 17 of 52
when the device is
BE
( )
×
q In N
V
OUT+
TO ADC
V
OUT–
ADT7460