AMC7820YSLASH2K datasheet(16/29 Pages) TI2

16 / 29 page

AMC7820

SBAS231B

www.ti.com

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 AVDD (+5.0V) rail will be

clamped if the value of VREF 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 AVDD – 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

to the leakage current of ESD diodes which are connected

from each op amp input to AGND and AVDD. 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) • (CFILTER).

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. 300k

Ω) 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 RISET resistor (see

Figure 7). Thus, the thermistor current from pin

THERM_I_OUTPUT (pin 44), ITHERM, is given by:

THERM

ISET

•

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, RISET 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.

FIGURE 3. Reference Circuitry.

FILTER

2.5V

REF

OUT

To ADC

2.5V

Bandgap

Reference

10k

Ω

To DACs

AMC7820Y/2K Datasheet(PDF) 16 Page - Texas Instruments

AMC7820Y/2K Datasheet(HTML) 16 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

Datasheet Download

Link URL