ADN8834ACPZ-R2 datasheet(19/27 Pages) AD

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Data Sheet

ADN8834

Rev. B | Page 19 of 27

Calculate RX using the following equation:













−

MID

HIGH

LOW

HIGH

LOW

HIGH

MID

LOW

THERMISTOR AMPLIFIER (CHOPPER 1)

The Chopper 1 amplifier can be used as a thermistor input

amplifier. In Figure 33, the output voltage is a function of the

thermistor temperature. The voltage at OUT1 is expressed as:

REF

OUT1













−

where:

RTH is a thermistor.

RX is a compensation resistor.

Calculate R using the following equation:

R = RX + RTH_@_25°C

VOUT1 is centered around VREF/2 at 25°C. An average temperature-

to-voltage coefficient is −25 mV/°C at a range of 5°C to 45°C.

–15

2.5

0.5

1.0

1.5

2.0

TEMPERATURE (°C)

Figure 34. VOUT1 vs. Temperature

PID COMPENSATION AMPLIFIER (CHOPPER 2)

Use the Chopper 2 amplifier as the PID compensation amplifier.

The voltage at OUT1 feeds into the PID compensation amplifier.

The frequency response of the PID compensation amplifier is

dictated by the compensation network. Apply the temperature

set voltage at IN2P. In Figure 39, the voltage at OUT2 is

calculated using the following equation:

)

(

TEMPSET

OUT1

TEMPSET

OUT2

−

where:

VTEMPSET is the control voltage input to the IN2P pin.

Z1 is the combination of RI, RD, and CD (see Figure 35).

Z2 is the combination of RP, CI, and CF (see Figure 35).

The user sets the exact compensation network. This network

varies from a simple integrator to proportional-integral (PI), PID

(proportional-integral-derivative), or any other type of network.

The user also determines the type of compensation and component

values because they are dependent on the thermal response of the

object and the TEC. One method to empirically determine these

values is to input a step function to IN2P; thus changing the target

temperature, and adjust the compensation network to minimize

the settling time of the TEC temperature.

A typical compensation network for temperature control of a laser

module is a PID loop consisting of a very low frequency pole and

two separate zeros at higher frequencies. Figure 35 shows a simple

network for implementing PID compensation. To reduce the noise

sensitivity of the control loop, an additional pole is added at a higher

frequency than that of the zeros. The bode plot of the magnitude is

shown in Figure 36. Use the following equation to calculate the

unity-gain crossover frequency of the feed-forward amplifier:

TECGAIN

0dB













−

2π

To ensure stability, the unity-gain crossover frequency must be

lower than the thermal time constant of the TEC and thermistor.

However, this thermal time constant is sometimes unspecified,

making it difficult to characterize. There are many texts written

on loop stabilization, and it is beyond the scope of this data sheet to

discuss all methods and trade-offs for optimizing compensation

networks.

VOUT1 is a convenient measure to gauge the thermal instability of

the system, which is also known as TEMPOUT. If the thermal loop

is in steady state, the TEMPOUT voltage equals the TEMPSET

voltage, meaning that the temperature of the controlled object

equals the target temperature.

OUT1

IN2N

OUT2

PID COMPENSATOR

CHOPPER 2

IN2P

ADN8834

VTEMPSET

Figure 35. Implementing a PID Compensation Loop

FREQUENCY

(Hz Log Scale)

0dB

2π × RICI

2π × RICD

2π × RPCI

2π × CD (RD + RI)

RD || RI

Figure 36. Bode Plot for PID Compensation

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