MAX11359A_12 Datasheet, PDF (31/67 Pages) Maxim Integrated Products – 16-Bit Data-Acquisition System with ADC, DAC, UPIOs, RTC, Voltage Monitors, and Temp Sensor

MAX11359A
16-Bit Data-Acquisition System with ADC, DAC,
UPIOs, RTC, Voltage Monitors, and Temp Sensor
Two-Current Method
For the two-current method, currents I₁ and I₂ are
passed through a p-n junction. This requires two V_BE
measurements. Temperature measurements can be
performed using I₁ and I₂.
( ) _TMEAS
=
q V_BE2 − V_BE1
nk
⎛
ln_⎝⎜
I₂
I₁
⎞
⎠⎟
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 I₁ and I₂:
1) Current I₁ 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 V₁.
3) Repeat steps 1 and 2 with I₂. I₁ is typically 4µA and
I₂ 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:
T_MEAS
=
q(N_VBE2 − N_VBE1)
nk
⎛
ln_⎝⎜
N_V2
N_V1
−
−
N_VBE2
N_VBE1
⎞
⎠⎟
×
V_REF
2¹⁶
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; I₁, I₂, M₁I₁, and M₂I₂. If
the currents are selected so (M₁ - 1)I₁ = (M₂ - 1)I₂, the
effect of the series resistance is eliminated from the
temperature measurements. For the currents I₁ = 4µA
and I₂ = 60µA, the factors are selected as M₁ = 16 and
M₂ = 2. This results in the currents I₃ = M₁I₁ = 64µA
and I₄ = M₂I₂ = 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 I₁ 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 I₂, I₃, and I₄.
The measured temperature is defined as follows:
( ) ( ) _TMEAS
=
q
N_VBE3
− N_VBE1 − q N_VBE4
⎛
nkIn_⎝⎜
M₁
M₂
⎞
⎠⎟
− N_VBE2
×
V_REF
2¹⁶
where V_REF is the reference voltage used and:
M₁
M₂
=
⎛
⎝⎜
N_V3
N_V1
−
−
N_VBE3
N_VBE1
⎞
⎠⎟
⎛
⎝⎜
N_V2
N_V4
− N_VBE2
− N_VBE4
⎞
⎠⎟
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:
⎛
⎞
T_ACTUAL
= T_MEAS
−
⎜
⎜
⎜
⎜
⎝
q(N_V2 − N_VBE2) − q(N_V1 − N_VBE1)
⎛
nkIn_⎝⎜
N_V2
N_V1
−
−
N_VBE2
N_VBE1
⎞
⎠⎟
×
V_REF
2¹⁶
×
R_S
R
⎟
⎟
⎟
⎟
⎠
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:
Maxim Integrated
31