whether they are used or not. The DXN input is biased
at one V
BE
above ground by an internal diode to set up
the ADC inputs for a differential measurement.
Resistance in series with the remote diode causes
about +1/2°C error per ohm.
A/D Conversion Sequence
A conversion sequence consists of a local temperature
measurement and a remote temperature measurement.
Each time a conversion begins, whether initiated auto-
matically in the free-running autoconvert mode
(RUN/STOP = 0) or by writing a “one-shot” command,
both channels are converted, and the results of both
measurements are available after the end of conver-
sion. A BUSY status bit in the Status register shows that
the device is actually performing a new conversion. The
results of the previous conversion sequence are still
available when the ADC is busy.
Remote-Diode Selection
The MAX6657/MAX6658/MAX6659 can directly mea-
sure the die temperature of CPUs and other ICs that
have on-board temperature-sensing diodes (see
Typical Operating Circuit
) or they can measure the tem-
perature of a discrete diode-connected transistor. The
type of remote diode used is set by bit 5 of the
Configuration Byte. If bit 5 is set to zero, the remote
sensor is a diode-connected transistor, and if bit 5 is set
to 1, the remote sensor is a substrate or common collec-
tor PNP transistor. For best accuracy, the discrete tran-
sistor should be a small-signal device with its collector
and base connected together. Accuracy has been
experimentally verified for all the devices listed in Table 1.
The transistor must be a small-signal type with a rela-
tively high forward voltage; otherwise, the A/D input
voltage range can be violated. The forward voltage at
the highest expected temperature must be greater than
0.25V at 10µA, and at the lowest expected tempera-
ture, forward voltage must be less than 0.95V at 100µA.
Large power transistors must not be used. Also, ensure
that the base resistance is less than 100Ω. Tight speci-
fications for forward current gain (50 < β < 150, for
example) indicate that the manufacturer has good
process controls and that the devices have consistent
V
BE
characteristics.
Thermal Mass and Self-Heating
When sensing local temperature, these devices are
intended to measure the temperature of the PC board
to which they are soldered. The leads provide a good
thermal path between the PC board traces and the die.
Thermal conductivity between the die and the ambient
air is poor by comparison, making air temperature mea-
surements impractical. Because the thermal mass of
the PC board is far greater than that of the MAX6657/
MAX6658/MAX6659, the devices follow temperature
changes on the PC board with little or no perceivable
delay.
When measuring the temperature of a CPU or other IC
with an on-chip sense junction, thermal mass has virtu-
ally no effect; the measured temperature of the junction
tracks the actual temperature within a conversion cycle.
When measuring temperature with discrete remote sen-
sors, smaller packages (i.e., a SOT23) yield the best
thermal response times. Take care to account for ther-
mal gradients between the heat source and the sensor,
and ensure that stray air currents across the sensor
package do not interfere with measurement accuracy.
Self-heating does not significantly affect measurement
accuracy. Remote-sensor self-heating due to the diode
current source is negligible. For the local diode, the
worst-case error occurs when autoconverting at the
fastest rate and simultaneously sinking maximum cur-
rent at the ALERT output. For example, with V
CC
=
+5.0V, a 16Hz conversion rate and ALERT sinking
1mA, the typical power dissipation is:
V
CC
x 450µA + 0.4V x 1mA = 2.65mW
θ
J-A
for the 8-pin SO package is about +170°C/W, so
assuming no copper PC board heat sinking, the result-
ing temperature rise is:
∆T = 2.65mW x +170°C/W = +0.45°C
Even under these engineered circumstances, it is diffi-
cult to introduce significant self-heating errors.
ADC Noise Filtering
The integrating ADC used has good noise rejection for
low-frequency signals such as 60Hz/120Hz power-sup-
ply hum. In noisy environments, high-frequency noise
reduction is needed for high-accuracy remote mea-
MAX6657/MAX6658/MAX6659
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