MAX1358B Maxim Integrated Products, MAX1358B Datasheet - Page 33

• Manufacturer Part Number: MAX1358B
• Description: Data-Acquisition System
• Manufacturer: Maxim Integrated Products

DataSheet.in
The temperature equations for the two-current and four-
current methods are as follows:
Two-current method:
Four-current method:
where T is the temperature in degrees Kelvin, V
the base to emitter voltage at current X, V
age across the current-sensing resistor at current X, q
is the charge on an electron, k is Boltzmann’s constant,
and n is the ideality factor for the diode. From a practi-
cal standpoint, it is easiest to combine all the constants
into one constant that also includes the voltage resolu-
tion of the ADC in unipolar mode. This requires intro-
ducing the term V
the ADC. An N prefix on a term indicates that it is the
integer value read directly from the ADC.
Two-current method:
Four-current method:
The natural log function (ln) is eliminated from the cal-
culation by using an approximation. Due to the small
part-to-part variation in current ratios, this approxima-
tion is extremely accurate.
Two-current method without an ln function:
Four-current method without an ln function:
16-Bit, Data-Acquisition System with ADC, DACs,
UPIOs, RTC, Voltage Monitors, and Temp Sensor
T = 0.1771 x V
N
T = q(V
VBE4
T = 0.1771 x V
T = 0.1771 x V
T = 0.1771 x V
2_(N
)/(2.0794 + 2(N
T = q(V
N
BE2
VBE4
(N
VR2
VR2
+ V
)/ln(N
/N
BE2
REF
REF
______________________________________________________________________________________
BE3
V
VR1
x N
REF
REF
REF
R3
(N
- V
, which is the reference voltage of
VR2
VR3
)/(V
(N
((N
- V
VBE2
- 15)/(N
(N
BE1
VR2
VBE2
x N
VBE2
BE1
/N
R1
VBE2
)/(n k ln(V
VR1
- N
x N
x V
VR3
- V
– N
VR2
+ N
VBE1
+ N
/N
VR3
R4
/N
BE4
VBE1
VR4
))
/N
VR1
VBE3
VBE3
/N
)/(n k ln((V
)/ln(N
VR1
R2
VR1
+ 8)
/N
)/(2.7081 +
/V
- N
- N
VR4
+ 15)
R1
/N
RX
VR2
VBE1
VBE1
VR4
))
)
is the volt-
/N
R2
VR1
- 8)/
-
BEX
-
x
)
is
q = electron charge = 1.60219 x 10
n = diode ideality = 1.000 (typ)
k = Boltzmann's constant = 1.3807 x 10
I1 = Nominal current (4µA)
I2 = Nominal current ng (60µA)
I3 = Nominal current (64µA)
I4 = Nominal current (120µA)
To convert the measured temperature in Kelvin to
degrees Celsius, the following formula is used:
For the external temperature measurement, a transistor
such as the 2N3904 is recommended.
An internal 1.25V bandgap reference has a buffer with
a selectable 1.0V/V, 1.638V/V, or 2.0V/V gain, result-
ing in nominally 1.25V, 2.048V, or 2.5V reference volt-
age at REF. The ADC and DACs use this reference
voltage. The state of the internal voltage reference
output buffer at POR is disabled so it can be driven, at
REF, with an external reference between AGND and
AV
ance of ±1%. Program the reference buffer through
the serial interface. Bypass REF with a 4.7µF capaci-
tor to AGND.
The MAX1358B includes one op amp. The op amp fea-
tures rail-to-rail outputs, near rail-to-rail inputs, and has
an 80kHz (1nF load) input bandwidth. The DACA_OP
(DACB_OP) register controls the power state of the op
amps. When powered down, the outputs of the op
amps is high impedance.
DD
. The MAX1358B reference has an initial toler-
Voltage Reference and Buffer
°C = K - 273.15
Uncommitted Operational
Amplifiers (Op Amps)
-19
coulombs
-23
Joules/Kelvin
33