ADA4530-1ARZ-R7 datasheet(33/52 Pages) AD

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Data Sheet

ADA4530-1

HIGH IMPEDANCE MEASUREMENTS

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Rev. C | 33 of 52

error level is 10%, the minimum measurable signal current is 2.25

pA for a circuit operating at 125°C.

ISRC = IB−(1/err – 1)

(6)

where err is the error level.

2.25pA=250fA 10.1 −1

INPUT RESISTANCE

The input resistance of the amplifier is another error source that

must be considered. Input resistance typically has two components:

differential and common mode. The differential input resistance is

suppressed by the negative feedback of the circuit. The ADA4530-1

has enough gain that the differential input resistance is much too

large to measure. The common-mode input resistance (hereafter

referred to as input resistance) is a more important error source.

The input resistance is equal to the change in input bias current

relative to the change in input voltage. This change is not caused

by a physical resistance inside the ADA4530-1; it is the result of

a complex relationship between the accuracy of the guard voltage

across the ESD structures and the input common-mode voltage;

that is, the input resistance changes with common mode voltage.

It is also possible for the input resistance to be negative. Negative

input resistance means the input bias current decreases as the

common-mode voltage increases.

The input resistance, RIN, can be approximated by calculating the

slope of the input bias current vs. common-mode voltage graphs

(see Figure 22 to Figure 33). For example, the noninverting input

resistance can be calculated at 125°C from Figure 32. The input

bias current changes by approximately 20 fA for common-mode

voltages from 4 V to 6 V.

RIN=ΔVCMΔIB+

(7)

RIN= 2V20fA=100TΩ

The slope of the curves in the input bias current vs. common-mode

voltage graphs increases rapidly outside the preferred common-

mode range (see Figure 22 to Figure 33). The input resistance

drops rapidly outside this range. This drop in input resistance must

be considered before operating these circuits with input voltages

close to the V− power supply.

Like the input bias current, the input resistance has a strong

temperature dependence. At lower temperatures, the amplifier input

resistance is so high that it is dominated by other error sources. It is

important to recognize the limitations of calculating input resistance

at lower temperatures. Measurement uncertainties make it difficult

to accurately calculate the ΔIB term. Consider the 85°C input bias

current vs. common-mode voltage graphs (see Figure 22 to Figure

27): the measurement uncertainties are equal to a few fA, which

is the same magnitude as the input bias current itself. These uncer-

tainties make it impossible to calculate input resistances higher

than a few hundred teraohms.

The input resistance affects the buffer circuit by loading down the

voltage sensor. This resistance acts as a voltage divider so the

voltage measured by the amplifier is some fraction of the unloaded

voltage of the sensor. This voltage drop is calculated as follows:

VA=VSRC RIN

RIN+RSRC

(8)

Consider the previous example of a 100 GΩ sensor operating at

125°C. The 100 TΩ input resistance causes the measured voltage

to equal 99.9% of the actual voltage, a 0.1% gain error.

The input resistance has much less of an effect on the TIA circuit.

The input common-mode voltage does not change in this circuit;

therefore, the error created is vanishingly small. The input resist-

ance affects the noise gain of the circuit, which changes the input

offset voltage error (see the Photodiode Interface section for more

information).

INPUT OFFSET VOLTAGE

The input offset voltage of the amplifier affects the buffer circuit by

adding directly to the voltage output of the sensor. This error is

typically much smaller than other errors.

The input offset voltage affects the TIA circuit in a different manner.

The burden voltage of the TIA is equal to the input offset voltage.

This burden voltage appears between the A and B terminals. An

error current is created by applying this burden voltage across the

sensor shunt resistance. For sensors with low output resistances

such as photodiodes, this error can be significant. Consider a

sensor with a 1 GΩ output resistance. The 50 µV maximum offset

voltage of the ADA4530-1 creates a 50 fA error current.

INSULATION RESISTANCE

The ADA4530-1 has such low input bias current and such high

input resistance that the insulation resistance of the materials that

are used to construct the circuit is often the largest error source.

Any insulators with finite resistance that come in contact with the

high impedance conductor contribute to the error current. Some

examples include the PCB laminate material, cable, and connector

insulation.

The physical insulation resistance is distributed across the entire

contact surface of the high impedance conductor, and it can end

at several different conductors at different potentials. It is useful

to make a simple model where all of these resistance paths are

lumped into a single resistor. This lumped element is shown as

RSHUNT in the voltage buffer circuit (see Figure 105).

The insulation resistance affects the buffer circuit in the same way

as the amplifier input resistance. This resistance acts as a voltage

divider so that the voltage measured by the amplifier is some

fraction of the unloaded voltage of the sensor.

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