Datasheet

MAX11359A
16-Bit Data-Acquisition System with ADC, DAC,
UPIOs, RTC, Voltage Monitors, and Temp Sensor
31
Maxim Integrated
Two-Current Method
For the two-current method, currents I
1
and I
2
are
passed through a p-n junction. This requires two V
BE
measurements. Temperature measurements can be
performed using I
1
and I
2
.
where k is Boltzman’s constant and q is the absolute
value of the charge on electron. A four-measurement
procedure is adopted to improve accuracy by precisely
measuring the ratio of I
1
and I
2
:
1) Current I
1
is driven through the diode and the series
resistor R, and the voltage across the diode is mea-
sured as V
BE1
.
2) For the same current, the voltage across the diode
and R is measured as V
1
.
3) Repeat steps 1 and 2 with I
2
. I
1
is typically 4µA and
I
2
is typically 60µA (see Table 21).
Since only four integer numbers are accessible from the
ADC conversions at a certain voltage reference, the previ-
ous equation can be represented in the following manner:
where N
V1
, N
V2
, N
VBE1
, and N
VBE2
are the measure-
ment results in integer format and V
REF
is the reference
voltage used in the ADC measurements.
Four-Current Method
The four-current method is used to account for the
diode series resistance and trace resistance. The four
currents are defined as follows; I
1
, I
2
, M
1
I
1
, and M
2
I
2
. If
the currents are selected so (M
1
- 1)I
1
= (M
2
- 1)I
2
, the
effect of the series resistance is eliminated from the
temperature measurements. For the currents I
1
= 4µA
and I
2
= 60µA, the factors are selected as M
1
= 16 and
M
2
= 2. This results in the currents I
3
= M
1
I
1
= 64µA
and I
4
= M
2
I
2
= 120µA (typ). As in the case of the two-
current method, two measurements per current are
used to improve accuracy by precisely measuring the
values of the currents.
1) Current I
1
is driven through the diode and the series
resistor R, and the voltage is measured across the
diode using the ADC as NVBE1.
2) For the same current, the voltage across the diode and
the series resistor is measured by the ADC as NV1.
3) Repeat steps 1 and 2 with I
2
, I
3
, and I
4
.
The measured temperature is defined as follows:
where V
REF
is the reference voltage used and:
External Temperature Sensor
For an external temperature sensor, either the two-cur-
rent or four-current method can be used. Connect an
external diode (such as 2N3904 or 2N3906) between
pins AIN1 and AGND (or AIN2 and AGND). Connect a
sense resistor R between AIN1 and AIN2. Maximize R
so the IR drop plus V
BE
of the p-n junction [(R x
I
MAX
)+V
BE
] is the smaller of the ADC reference voltage
or (AVDD - 400mV). The same procedure as the inter-
nal temperature sensor can be used for the external
temperature sensor, by routing the currents to AIN1 (or
AIN2) (see Table 20).
For the two-current method, if the external diode’s
series resistance (R
S
) is known, then the temperature
measurement can be corrected as shown below:
Temperature-Sensor Calibration
To account for various error sources during the temper-
ature measurement, the internal temperature sensor is
calibrated at the factory. The calibrated temperature
equation is:
TT
qN N qN N
nkIn
NN
NN
VR
R
ACTUAL MEAS
V VBE V VBE
V VBE
V VBE
REF S
=−
−−−
−
−
⎛
⎝
⎜
⎞
⎠
⎟
××
⎛
⎝
⎜
⎜
⎜
⎜
⎞
⎠
⎟
⎟
⎟
⎟
()()
2211
22
11
16
2
M
M
NN
NN
NN
NN
V VBE
V VBE
V VBE
V VBE
1
2
33
11
22
44
=
−
−
⎛
⎝
⎜
⎞
⎠
⎟
−
−
⎛
⎝
⎜
⎞
⎠
⎟
T
qN N qN N
nkIn
M
M
V
MEAS
VBE VBE VBE VBE
REF
=
−
()
−−
()
⎛
⎝
⎜
⎞
⎠
⎟
×
31 42
1
2
16
2
T
qN N
nk
NN
NN
V
MEAS
VBE VBE
V VBE
V VBE
REF
()
ln
=
−
−
−
⎛
⎝
⎜
⎞
⎠
⎟
×
21
22
11
16
2
T
qV V
nk
I
I
MEAS
BE BE
ln
=
−
()
⎛
⎝
⎜
⎞
⎠
⎟
21
2
1