ADA4530-1ARZ-R7 datasheet(40/51 Pages) AD

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Data Sheet

ADA4530-1

Rev. A | Page 39 of 50

CURRENT NOISE CONSIDERATIONS

The current noise from an amplifier input pin is important

when it flows through an impedance and generates a voltage

noise. If the current noise and impedance are large enough, the

resulting voltage noise can dominate the other noise sources in

the circuit such as the voltage noise of the resistors and amplifier.

For an electrometer amplifier such as the ADA4530-1, the

typical circuit impedances are so large that the current noise of

the amplifier can be the most important noise source.

To measure current noise, it is necessary to flow the noise

current through a test impedance large enough that the

resulting noise voltage is larger than the other noise voltages in

the circuit. Practically, this test impedance is usually a resistor.

All resistors have their own thermal noise. The value of thermal

noise is usually presented as an output referred voltage noise

spectral density (NSD), VNRTO.

VNRTO = √(4kTR)

where:

k is Boltzmann’s constant.

T is the temperature in Kelvin.

R is the resistance value.

The resistor thermal noise can be interpreted as a current NSD

by dividing the thermal noise with the resistance value, R, per

Ohm’s Law.

Table 8 shows the thermal noise of a series of resistor values

presented as both voltage and current noise. The current noise

of a resistor decreases as the resistance increases. This surprising

result illustrates that it is necessary to use high valued resistors

to measure low levels of current noise.

Table 8. Resistor Thermal Noise

Resistor Value

Voltage Noise

Current Noise

1 MΩ

128 nV/√Hz

128 fA/√Hz

100 MΩ

1.28 μV/√Hz

12.8 fA/√Hz

10 GΩ

12.8 μV/√Hz

1.28 fA/√Hz

1 TΩ

128 μV/√Hz

128 aA/√Hz

The measurement setup used to gather the current noise data is

shown in Figure 116. The ADA4530-1 is configured as a TIA

with a large value feedback resistor, RF. All amplifier current

noise from the inverting input flows through Resistor RF to

produce a voltage noise at VOUT.

ADA4530-1

SR785

DSA

VOUT

Figure 116. Current Noise Measurement Setup

The output referred voltage NSD, VNRTO, is sampled by the SR785

high performance dynamic signal analyzer (DSA) and is equal

to the root-sum-square of the amplifier current noise multiplied by

RF, the resistor thermal noise, and amplifier voltage noise.

VNRTO = √((IN−RF)2 + 4kTRF + VN2)

where:

IN− is the amplifier inverting current noise.

4kTRF is the resistor thermal noise.

VN2 is the amplifier voltage noise.

Calculate the current noise of the amplifier from VNRTO as follows:

NRTO

kTR







(1)

For Equation 1 to be valid, the measured noise must be somewhat

larger than the resistor thermal noise plus the amplifier voltage

noise. In practice, ensure that the resistor current noise is less

than or equal to the amplifier current noise. For example, if the

amplifier noise is expected to be 2 fA/√Hz, use a value of RF that

is at least 10 GΩ, according to Table 8.

The amplifier voltage noise is not a concern at most frequencies

because the resistor thermal noise is much larger than the amplifier

voltage noise. At very low frequencies, this assumption is not

valid due to the 1/f characteristic of the amplifier voltage noise.

It is important to consider the bandwidth limitations of the

current noise measurement system shown in Figure 116. The

presence of stray capacitance makes it impossible to maintain

the high impedances required for the measurement. All stray

capacitance that couple the amplifier output to the inverting

input can be lumped into a single capacitor, CF, as shown in

Figure 116.

The current noise must pass through RF to become voltage

noise. However, in practice, the current noise passes through

the parallel combination of RF and CF to become voltage noise.

At frequencies higher than the RFCF pole, most of the noise current

flows through the capacitor and current noise calculations at

these frequencies are error prone due to the distributed parasitic

nature of CF. A good guideline is to set the measurement band-

width limit equal to the RFCF pole frequency.

The measurement bandwidth limitations for high valued resistors

can be surprisingly low. Table 9 shows the −3 dB bandwidth of a

series of resistor values with a practical minimum stray capacitance

value.

Table 9. Bandwidth Limitations

Resistor Value

Capacitor Value

−3 dB Bandwidth

1 MΩ

100 fF

1.59 MHz

100 MΩ

100 fF

15.9 kHz

10 GΩ

100 fF

159 Hz

1 TΩ

100 fF

1.59 Hz

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