ADA4530-1ARZ-R7 datasheet(40/52 Pages) AD

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Data Sheet

ADA4530-1

CURRENT NOISE CONSIDERATIONS

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Rev. C | 40 of 52

Reconsider the example amplifier with 2 fA/√Hz of current noise.

The required RF value of 10 GΩ also limits the measurement

bandwidth to 159 Hz according to Table 9.

It is useful to construct a table that combines the resistor noise

and measurement bandwidth guidelines. Table 10 shows the ap-

proximate bandwidth limitations for a variety of input current noise

measurements.

Table 10. Measurement Current Noise Density vs. Bandwidth

Current Noise Density

Bandwidth

128 aA/√Hz

1.59 Hz

1.28 fA/√Hz

159 Hz

12.8 fA/√Hz

15.9 kHz

128 fA/√Hz

1.59 MHz

Table 10 demonstrates the error of the often claimed 0.1 fA/√Hz

at 10 kHz presented in the specifications of low input bias current

amplifiers. Measuring this value requires a 1 TΩ resistor with less

than 15.9 aF (15.9 × 10−18 F) of stray capacitance, which is

impossible.

These kinds of claims are simply shot noise calculations based on

the specified input bias currents of a few tens of femtoamperes.

Calculate the shot noise of a semiconductor as follows:

Shot Noise = √(2qIB)

(14)

where:

q is the charge on an electron.

IB is the current flowing through a junction.

Shot noise calculations are appropriate only for some legacy

JFET-based electrometer amplifiers, where only a single junction

is connected to the amplifier input pins. Modern high impedance

amplifiers have several semiconductor junctions connected to the

amplifier input pins. The most significant of these junctions are the

ESD diode structures. The input bias currents are equal to the sum

of these diode currents. The diode currents are designed to cancel

each other, but the shot noise currents are uncorrelated and cannot

cancel, which makes calculating the shot noise from the input bias

current impossible.

Even when appropriate, these shot noise calculations neglect all

capacitive coupling effects so that they are valid only at very low

frequencies. The gate to source capacitance of the input transistors

couples noise currents from sources other than the input junctions

for frequencies above a few tens of hertz. This blowback noise

effect is present in all amplifiers, and it ensures that the current

NSD always increases as frequency increases.

The complex relationship between current noise, feedback resist-

ance, and bandwidth means that the correct way to characterize

the current noise of an electrometer amplifier is by measuring the

output NSD with a variety of feedback resistors that cover the entire

span of values used in the end applications. Each feedback resistor

establishes a boundary for the minimum measurable current noise

over a range of frequencies.

It is critically important to use high quality resistors during this

measurement. Many high valued resistors designed for high voltage

operation are nonlinear at low voltage levels and are not suitable

for electrometer work. Inferior resistors can also have their own

1/f noise that corrupts the measurement results. Table 11 lists the

resistors used for the characterization of the ADA4530-1.

Table 11. Test Resistor Device Numbers

Resistor Value

Manufacturer

Device Number

100 MΩ

Vishay

RNX050100MDHLB

1 GΩ

Ohmite

RX-1M1007GE

10 GΩ

Ohmite

RX-1M1008JE

100 GΩ

Ohmite

RX-1M1009FE

1 TΩ

Ohmite

RX-1M100AKE

Figure 117 shows the output referred voltage NSD (VNRTO) of the

transimpedance test circuit for the test resistors listed in Table 11.

The calculated thermal noise for each resistor is represented with

the dashed line. The black dashed line represents the 1/f voltage

noise of the amplifier.

Figure 117. Transimpedance NSD Referred to Output

VNRTO is dominated by the resistor noise for all of the test resistors

up 1 TΩ. This means that the current noise contribution from the

ADA4530-1 is insignificant relative to the thermal noise of these

resistors.

It is possible to calculate the current noise of the ADA4530-1 with

the 1 TΩ resistor. This result is shown in Figure 118. It is impossible

to calculate the amplifier current noise for all of the other resistors,

because, at those resistor values, the resistor noise is much greater

than the amplifier current noise. The current noise densities of each

of the test resistors are plotted as dashed lines in Figure 118. The

current noise of the ADA4530-1 is below the resistor noise values.

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