Data Sheet

ADR1000

THEORY OF OPERATION

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Rev. B | 7 of 10

The ADR1000 consists of a buried Zener diode, a temperature

compensating transistor, a temperature sensing transistor, and a

Zener diode temperature coefficient is approximately +2 mV/°C,

−2 mV/°C. Referring to Figure 9, an external op amp (U3), in

combination with an external resistor (R1), is used to set the Zener

operating current as follows:

R

Iz

 − 7 Ω

where:

7 Ω is the bulk resistance to the Zener anode.

R

Iz

 − 7 Ω = 129.5 Ω

Note that because the 7 Ω bulk resistance (R0 in Figure 1) sche-

matically appears under the Q1 base, it must be included in the

Zener operating current is output voltage noise. The ADR1000 can

achieve a total output noise of 0.14 ppm (0.9 µV p-p) in the 0.1

with the dominant noise source being the Zener diode. Increasing

inverse square root of the Zener current. A Zener bias current

greater than 8 mA is not practical because power dissipation limits

maximum ambient temperature. The ADR1000 applications circuit

output noise spectral density has been measured over a range of

Zener set currents (see Figure 3). The ADR1000 long-term drift

SETTING THE OPERATING TEMPERATURE

The ADR1000 can regulate chip operating temperature to within

a few millidegrees over a 100°C ambient temperature change.

This means if the unheated reference temperature coefficient is

20 ppm/°C, then the theoretical heated temperature coefficient is

well below 0.1 ppm/°C. This performance is difficult to achieve in a

practical circuit (refer to the Avoiding Thermocouple Errors section

copy of the 6.62 V reference voltage (see Figure 9). The 13 kΩ:1

current to satisfy the condition that the input terminals of U2 must

be equal to within a few hundred microvolts. Thus, the noninverting

input of U2 is pulled up until its inputs clamp or until its noninverting

of U2 causes the output of U2 to pull up, increasing the amount of

power dissipated in the on-chip heater. Because the transistor base

emitter voltage has a negative temperature coefficient, the collector

current of Q2 increases as the chip temperature rises, causing the

op amp inputs to move closer together until the voltage drop across

R3 satisfies the loop. The temperature at which the thermal loop is

Figure 7 shows the ADR1000 and LTZ1000A total supply current

vs. the ambient temperature. Notice that the heater current has a

square root dependence on the difference between the ambient

temperature and the set temperature because the power dissipated

in the heater is proportional to the square of the current. When

the ambient temperature reaches the set temperature, the current

in the heater goes to zero, and the chip temperature is no longer

regulated.

THERMAL RESISTANCE

The ADR1000 uses a specialized epoxy die attachment to maxi-

mize the thermal isolation that reduces the power consumption

required to achieve a given set temperature. At an ambient temper-

ature of 10°C, the heater power consumption is approximately 35

the total supply current minus 5 mA for the Zener current and other

components on the PCB (see Figure 7). For 300 mW dissipation in

the heater, the internal temperature of the ADR1000 is elevated by

65°C, yielding a 216°C/W junction to ambient thermal impedance

Table 6. Recommended Values for Varying the Set Temperature in 5° Increments

Estimate Set Temperature (°C)

R4 (Ω)

R5 (kΩ)

80

13 k + 316

1

464

75

13 k

1

474

70

13 k − 316

1

484

65

13 k − 632

1

494