REV. A

ADM1028

–9–

7. For really long distances (up to 100 feet) use shielded twisted-

pair such as Belden #8451 microphone cable. Connect the

twisted pair to D+ and D– and the shield to GND close to

the ADM1028. Leave the remote end of the shield uncon-

nected to avoid ground loops.

10MIL

10MIL

10MIL

10MIL

10MIL

10MIL

10MIL

GND

D+

D–

GND

Figure 4. Arrangement of Signal Tracks

Because the measurement technique uses switched current

sources, excessive cable and/or filter capacitance can affect the

measurement. When using long cables, the filter capacitor C1

may be reduced or removed. In any case, the total shunt capaci-

tance should not exceed 1000 pF.

Cable resistance can also introduce errors. 1

Ω series resistance

introduces about 0.5

°C error.

ANALOG OUTPUT

The ADM1028 has a single analog output (FAN_SPD) from an

unsigned 8-bit DAC which produces 0 V–2.5 V. The analog

output register defaults to 00 during power-on reset, which pro-

duces minimum fan speed. The analog output may be amplified

and buffered with external circuitry such as an op amp and tran-

sistor to provide fan speed control.

Suitable fan drive circuits are given in Figures 5a to 5e. When

using any of these circuits, the following points should be noted:

1. All of these circuits will provide an output range from zero

2. To amplify the 2.5 V range of the analog output up to

3. Care must be taken when choosing the op amp to ensure

that its input common-mode range and output voltage swing

are suitable.

4. The op amp may be powered from the +V rail alone. If it is

powered from +V then the input common-mode range

should include ground to accommodate the minimum out-

put voltage of the DAC, and the output voltage should

swing below 0.6 V to ensure that the transistor can be

turned fully off.

greater than the maximum fan current, and be capable of

dissipating power due to the voltage dropped across it when

the fan is not operating at full-speed.

6. If the fan motor produces a large back e.m.f. when switched

off, it may be necessary to add clamp diodes to protect the

output transistors in the event that the output very quickly

goes from full-scale to zero.

Figure 5c shows how the

FAN_OFF signal may be used (with

any of the control circuits) to gate the fan on and off indepen-

dent of the value on the FAN_SPD/NTEST_IN pin.

FAN_SPD

5V

Q1

NDT452 P

5V

FAN

R1

10k

R2

15k

+

AD8541

Figure 5a. 5 V Fan Circuit with Op Amp

12V

Q1

BD136

2SA968

R1

10k

R2

39k

R3

1k

R4

1k

+

AD8519

FAN_SPD

Figure 5b. 12 V Fan Circuit with Op Amp and PNP

Transistor

12V

R1

10k

R2

39k

R3

100k

Q1

NDT452 P

R4

1k

3.3V

FAN_OFF

+

AD8519

FAN_SPD

Q2

MMFT305 5V

Figure 5c. 12 V Fan Circuit with Op Amp and

P-Channel MOSFET

12V

Q3

NDT452 P

R4

100k

R2

3.9k

R1

1k

R3

100k

FAN_SPD

R5

5k

Q1/Q2

MBT3904

DUAL

Figure 5d. Discrete 12 V Fan Drive Circuit with

P-Channel MOSFET, Single Supply

12V

Q4

BD132

TIP32A

R4

100k

R5

100k

FAN_SPD

R6

5k

Q1/Q2

MBT3904

DUAL

R2

3.9k

R1

1k

R3

100

Q3

BC556

2N3906

Figure 5e. Discrete 12 V Fan Drive Circuit with

Bipolar Output Single Supply