AMC7820_15 Datasheet, PDF (16/32 Pages) Texas Instruments – ANALOG MONITORING AND CONTROL

There are three other limitations when operating a DAC
output near the supply rails, even if the load current is very
small. 1) The output stage of the DAC buffer amplifier clamps
at about +5mV above AGND. See the typical characteristics
curves in the Operational Amplifier section for an illustration of
this behavior. 2) If a DAC buffer amplifier has a negative input
offset voltage, the output cannot increase until the input digital
code is sufficient to overcome this negative offset. 3) When
DAC gain is set at +2, swing near the AV_DD (+5.0V) rail will be
clamped if the value of V_REF is greater than +2.50V and/or if
the matching of the output buffer gain setting resistors gives a
GAIN that is greater than +2.0.
OPERATIONAL AMPLIFIER
The AMC7820 has nine operational amplifiers. OPA8 is used
to set the current source output. OPA9 buffers the tempera-
ture sensor voltage. All others can be configured for signal
conditioning or control function.
The outputs of OPA3, OPA4, OPA5, and OPA6 connect to
the ADC analog input channels 0, 1, 6, and 7, respectively.
The output of OPA7 connects to SW2_OUT when SW2 is
enabled. Therefore, SW2 enables or disables OPA7 if
SW2_OUT is used as the output of OPA7.
All amplifiers use a PMOS differential input stage that allows
the common-mode input to extend from 200mV below ground
up to AV_DD – 1.2V while maintaining very good CMRR, low
offset voltage, low noise, and good PSRR. High open-loop
gain provides excellent signal linearity and low closed-loop
output impedance.
The rail-to-rail output stage can swing to within a few milli-
volts of the supply rails provided the sink and source currents
are small (see the typical characteristics). Refer to the DAC
section for more information regarding swinging close to the
supply rails.
The input bias currents are very low, typically ranging from
less than 1pA to a few pA. This current is almost entirely due
2.5V
Bandgap
Reference
C_FILTER
27
10kΩ
A1
2.5V
REF_OUT
26
To DACs
A2
FIGURE 3. Reference Circuitry.
To ADC
to the leakage current of ESD diodes which are connected
from each op amp input to AGND and AV_DD. For each 10°C
rise in temperature, the leakage current will double, following
classic diode leakage current versus temperature. Input
offset current is the difference between the +IN and –IN bias
current. In other words, the offset current is a measurement
of the matching of the two input bias currents. Refer to the
typical characteristics for more detail.
REFERENCE
The AMC7820 has an internal +2.5V bandgap voltage refer-
ence, as shown in Figure 3. Buffered by A1, the reference
voltage is available on pin 26. The reference circuit can be
overdriven by an external reference on pin 27. This pin also
provides a point for filtering the internal reference, if desired
a capacitor may be placed from pin 27 to analog ground to
decrease reference noise. The time constant of this filter is
(10K) • (C_FILTER).
The internal reference voltage can be adjusted by providing
a small current into or out of pin 27. This current can be
generated by a large resistor (e.g. 300kΩ) connected from
pin 27 to an adjustable voltage source, such as the wiper of
a potentiometer or the output of a DAC.
THERMISTOR CURRENT SOURCE AND
TEMPERATURE SENSOR VOLTAGE
The thermistor current source output is set by the resistor
connected from the ISET_RESISTOR, pin 45, to ground. The
+2.5V reference voltage is forced on pin 45 by the closed-
loop action of OPA8. The actual thermistor current is pro-
vided by a 1:4 current mirror that provides a current drive of
4 times the current flowing through the R_ISET resistor (see
Figure 7). Thus, the thermistor current from pin
THERM_I_OUTPUT (pin 44), I_THERM, is given by:
I_THERM
=
2.5V
R_ISET
•4
The thermistor is driven by the current coming out of
THERM_I_OUTPUT. The voltage developed across the ther-
mistor is then buffered by the unity-gain buffer amplifier (OPA9).
The voltage on pin T_SENSOR_VOLTAGE represents the ac-
tual TEC temperature.
For best performance, R_ISET should have a TCR of 10ppm/°C or
less, and a tolerance of 0.1%. Using a current source preserves
the full sensitivity of the thermistor (typically 50mV/°C). If a
resistor were used to power the thermistor, the sensitivity would
be reduced to approximately 25mV/°C, due to the voltage
divider created by such an arrangement.
Because the thermistor current source has high output imped-
ance and wide voltage compliance, it is possible for two
separate laser modules to be controlled by connecting their
thermistors in series with the output of this current source.
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AMC7820
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