AD743

REV. C

–7–

OP AMP PERFORMANCE: JFET VS. BIPOLAR

The AD743 is the first monolithic JFET op amp to offer the low

input voltage noise of an industry-standard bipolar op amp

without its inherent input current errors. This is demonstrated

in Figure 24, which compares input voltage noise vs. input

source resistance of the OP27 and the AD743 op amps. From

this figure, it is clear that at high source impedance the low

current noise of the AD743 also provides lower total noise. It is

also important to note that with the AD743 this noise reduction

extends all the way down to low source impedances. The lower

dc current errors of the AD743 also reduce errors due to offset

and drift at high source impedances (Figure 25).

100

1k

10k

100k

1

10

100

1000

1M

10M

SOURCE RESISTANCE –

Ω

OP27 &

RESISTOR

AD743 + RESISTOR

RESISTOR NOISE ONLY

AD743 & RESISTOR

OR

OP27 & RESISTOR

O

E

(

)

(– – –)

( — )

Figure 24. Total Input Noise Spectral Density @ 1 kHz vs.

Source Resistance

SOURCE RESISTANCE –

Ω

ADOP27G

AD743 KN

100

10

1.0

0.1

100

1k

10k

100k

1M

10M

Figure 25. Input Offset Voltage vs. Source Resistance

DESIGNING CIRCUITS FOR LOW NOISE

An op amp’s input voltage noise performance is typicaly divided

into two regions: flatband and low frequency noise. The AD743

offers excellent performance with respect to both. The figure of

2.9 nV/

√Hz @ 10 kHz is excellent for JFET input amplifier.

The 0.1 Hz to 10 Hz noise is typically 0.38

µV p-p. The user

should pay careful attention to several design details in order to

optimize low frequency noise performance. Random air currents

can generate varying thermocouple voltages that appear as low

frequency noise: therefore sensitive circuitry should be well

shielded from air flow. Keeping absolute chip temperature low

also reduces low frequency noise in two ways: first, the low

frequency noise is strongly dependent on the ambient

temperature and increases above +25

°C. Secondly, since the

gradient of temperature from the IC package to ambient is

greater, the noise generated by random air currents, as

previously mentioned, will be larger in magnitude. Chip

temperature can be reduced both by operation at reduced

supply voltages and by the use of a suitable clip-on heat sink, if

possible.

Low frequency current noise can be computed from the

magnitude of the dc bias current (

~

below approximately 100 Hz with a 1/f power spectral density.

For the AD743 the typical value of current noise is 6.9 fA/

√Hz

at 1 kHz. Using the formula,

~

4kT /R

∆f , to compute the

Johnson noise of a resistor, expressed as a current, one can see

that the current noise of the AD743 is equivalent to that of a

3.45

10

8

Ω source resistance.

At high frequencies, the current noise of a FET increases

proportionately to frequency. This noise is due to the “real” part

of the gate input impedance, which decreases with frequency.

This noise component usually is not important, since the voltage

noise of the amplifier impressed upon its input capacitance is an

apparent current noise of approximately the same magnitude.

In any FET input amplifier, the current noise of the internal

bias circuitry can be coupled externally via the gate-to-source

capacitances and appears as input current noise. This noise is

totally correlated at the inputs, so source impedance matching

will tend to cancel out its effect. Both input resistance and input

capacitance should be balanced whenever dealing with source

capacitances of less than 300 pF in value.

LOW NOISE CHARGE AMPLIFIERS

As stated, the AD743 provides both low voltage and low current

noise. This combination makes this device particularly suitable

in applications requiring very high charge sensitivity, such as

capacitive accelerometers and hydrophones. When dealing with

a high source capacitance, it is useful to consider the total input

charge uncertainty as a measure of system noise.

Charge (Q) is related to voltage and current by the simply stated

fundamental relationships:

Q

= CV and I = dQ

dt

As shown, voltage, current and charge noise can all be directly

related. The change in open circuit voltage (

∆V) on a capacitor

will equal the combination of the change in charge (

∆Q/C) and

the change in capacitance with a built in charge (Q/

∆C).