ADA4530-1ARZ-R7 datasheet(39/52 Pages) AD

39 / 52 page

Data Sheet

ADA4530-1

CURRENT NOISE CONSIDERATIONS

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Rev. C | 39 of 52

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 volt-

age is larger than the other noise voltages in the circuit. Practically,

this test impedance is usually a resistor. All resistors have thermal

noise. The value of thermal noise is usually presented as an output

referred voltage noise spectral density (NSD), VNRTO.

VNRTO = √(4kTR)

(11)

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.

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)

(12)

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:

IN−= VNRTO2−4kTRF−VN2

(13)

For Equation 13 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 capacitances

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 bandwidth 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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