ADN8831

Data Sheet

Rev. A | Page 16 of 20

–15

5

25

45

0

2.5

65

0.5

1.0

1.5

2.0

TEMPERATURE(°C)

PID COMPENSATION AMPLIFIER (Chop2)

Use the Chop2 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 17, the voltage at OUT2 is calcu-

lated using the following equation:

)

(

TEMPSET

OUT1

TEMPSET

OUT2

V

V

Z1

Z2

V

V







The user sets the exact compensation network. This network

varies from a simple integrator to PI, PID, 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 for empirically

determining these values is to input a step function to IN2P,

therefore changing the target temperature, and adjusting 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 19

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 the zeros. The bode

plot of the magnitude is shown in Figure 20. The unity-gain

crossover frequency of the feedforward amplifier is calculated

using the following equation:

TECGAIN

R3C1

f









80

2

1

dB

0

To ensure stability, the unity-gain crossover frequency is to 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 in optimizing

compensation networks.

ADN8831

CHOP2

IN2P

IN2N

4

7

6

OUT1

OUT2

5

C1

CF

C2

R2

R3

R1

Figure 19. Implementing a PID Compensation Loop

FREQUENCY (Hz Log Scale)

0dB

1

2πR3C1

R1

R3

1

2πR3C2

1

2πR1C1

1

2πC2 (R2 + R3)

R1

R2 || R3

Figure 20. Bode Plot for PID Compensation

With an ADN8831-EVALZ board, AN-695, an application note

shows how to determine the PID network components for a

stable TEC subsystem performance.