ADA4530-1ARZ-R7 datasheet(34/51 Pages) AD

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Data Sheet

ADA4530-1

Rev. A | Page 33 of 50

The magnitude of the input bias current limits how small of a

signal current may be resolved accurately. For example, if the

acceptable 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)

where err is the error level.

















250

INPUT RESISTANCE

The input resistance of the amplifier is another error source that

must be considered. Input resistance typically has two compo-

nents: 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. Note

that the input bias current changes by approximately 20 fA for

common-mode voltages from 4 V to 6 V.







TΩ

100



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 uncertain-

ties 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 uncertainties make it impossible to calcu-

late 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:

SRC





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 resistance 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 printed circuit board

(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

may 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. This error

is significant because it is very difficult to maintain high

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