AD8671 Просмотр технического описания (PDF) - Analog Devices

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AD8671

Precision, Very Low Noise, Low Input Bias Current Operational Amplifiers

Analog Devices 

APPLICATIONS

UNITY-GAIN FOLLOWER APPLICATIONS

When large transient pulses (>1 V) are applied at the positive

terminal of amplifiers (such as the OP27, LT1007, OPA227, and

AD8671) with back-to-back diodes at the input stage, the use of

a resistor in the feedback loop is recommended to avoid having

the amplifier load the signal generator. The feedback resistor,

RF, should be at least 500 Ω. However, if large values must be

used for RF, a small capacitor, CF, should be inserted in parallel

with RF to compensate for the pole introduced by the input

capacitance and RF.

Figure 32 shows the uncompensated output response with a

10 kΩ resistor in the feedback and the compensated response

with CF = 15 pF.

OUTPUT UNCOMPENSATED

OUTPUT

COMPENSATED

REF1 +OVER

23.23%

CH2 +OVER

7.885%

TIME (100ns/DIV)

Figure 32. Transient Output Response

OUTPUT PHASE REVERSAL

Phase reversal is a change of polarity in the amplifier transfer

function that occurs when the input voltage exceeds the supply

voltage. The AD8671/AD8672/AD8674 do not exhibit phase

reversal even when the input voltage is 1 V beyond the supplies.

AD8671/AD8672/AD8674

VSY = ±15V

VIN

VOUT

TIME (10μs/DIV)

Figure 33. Output Phase Reversal

TOTAL NOISE VS. SOURCE RESISTANCE

The low input voltage noise of the AD8671/AD8672/AD8674

makes them a great choice for applications with low source

resistance. However, because they have low input current noise,

they can also be used in circuits with substantial source

resistance.

Figure 34 shows the voltage noise, current noise, thermal noise,

and total rms noise of the AD8671 as a function of the source

resistance.

For RS < 475 Ω, the input voltage noise, en, dominates.

For 475 Ω < RS < 412 kΩ, thermal noise dominates.

For RS > 412 kΩ, the input current noise dominates.

1000

100

10

1

10

C

in

en_t

(4kRST)1/2

en

A

B

100

1k

10k

100k

1M

SOURCE RESISTANCE (Ω)

Figure 34. Noise vs. Source Resistance

Rev. C | Page 11 of 16

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