Data Sheet

ADR1000

THEORY OF OPERATION

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Rev. B | 8 of 10

HEATER HEADROOM CONSIDERATIONS

When colder ambient temperatures are combined with higher set

amp driving its base may need more headroom. For example, at

−40°C, the heater current is approximately 45 mA, which, when

multiplied by the 242 Ω heater resistance, raises Pin 1 (HTR+) to

10.9 V. A positive supply voltage (V+) set at 15 V results in 4.1 V

the temperature coefficient of the ADR1000 at lower ambient tem-

This issue can be solved by either increasing V+ or by choosing a

where:

discrete transistor dependent.

where β is the transistor current gain, assumed to be 100.

Figure 8 shows a plot of the approximate minimum heater supply

Figure 8. Minimum Supply Voltage vs. Required Temperature Rise

CHOOSING EXTERNAL OP AMPS

The ADA4084-x op amp is used with the ADR1000 evaluation

board because it has a common-mode range that includes ground

on a single supply, and because the output swings rail to rail.

Ground sensing inputs ensure proper startup when input signals

are both close to 0 V. Post startup, the ADA4084-x common mode

remains at around 480 mV. Rail-to-rail output swing ensures that

the heater voltage can be driven from 0 V to 1 V below the positive

supply. One additional requirement is that the op amp not swing to

exactly 0 V during startup, which creates a stable operating point of

0 V on the reference output. Because the ADA4084-x is a bipolar

amplifier, its negative output swing is limited by the typical output

output from reaching true ground.

Other than proper startup, most op amps have little to no effect

on the ADR1000 reference specification. Op amp parameters such

as offset and noise do not significantly affect the performance of

the ADR1000 applications circuit. Referring to Figure 9, Q1 runs

at a gain of approximately 230, which acts as an additional gain

stage in between the Zener reference and the amplifier. This stage

effectively attenuates any noise or offset contribution by 230:1.

CHOOSING EXTERNAL RESISTORS

Although the ADR1000 schematic reduces the external resistance

sensitivity by a factor of 200 or more, care must be taken in

selecting certain components. For example, if a resistor exhibited

200 ppm long-term drift, that affects the reference output by only

1 ppm. In absolute terms, 1 ppm is a small value but significant

compared to the relative stability of the ADR1000. Resistors R1,

R2, R4, and R5 are metal foil, wire wound, or precision thin film to

minimize noise, long-term drift, and temperature coefficient. Table 7

shows the ratio of resistor change to reference voltage change.

Table 7. Application Circuit Resistor Attenuation Factors

Component

Attenuation Factor

R1

340:1

R2

230:1

R3

R4, R5

tempco to vary by 10/340 = 0.03 ppm/⁰C. Also, if R1 exhibits a

long-term drift of 40 ppm, it causes a 40/340 = 0.12 ppm change in

reference voltage.

The unheated tempco condition for R3, R4, and R5 is included

in Table 7 because those resistors thermostatically control IC

temperature. Therefore, the unheated drift of the reference loop

is significant. The R4 and R5 tempcos must track to prevent any

dependence of the die set temperature on ambient temperature.