ADA4530-1ARZ-R7 datasheet(32/52 Pages) AD

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Data Sheet

ADA4530-1

HIGH IMPEDANCE MEASUREMENTS

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Rev. C | 32 of 52

The ADA4530-1 is designed to maximize the performance of very

high impedance circuits. Its performance advantages make it useful

for circuit impedances ranging from 100 MΩ to over 10 TΩ. Meas-

urements of high impedance circuits are subject to a number of

error sources. General information about making measurements

from high resistance sources can be found in the Low Level

Measurements Handbook, sixth edition (Keithley Instruments, Inc.,

2004).

The ADA4530-1 is typically used in two kinds of circuits: a buffer

and a transimpedance amplifier (TIA). Buffer circuits are useful

for measuring voltage output sensors with high output resistance.

Some example sensors include pH probes and reference electro-

des (RE) in coulometry control loops. TIA circuits are useful for

converting the signal from a current output sensor to an output

voltage. Some example sensors include photodiodes and ion cham-

bers.

The following sections describe some of the most important error

sources when using the ADA4530-1 in these circuits. Simplified

models with error sources are provided for the buffer (see Figure

105) and the TIA (see Figure 106).

The buffer circuit models the voltage output sensor as a voltage

source (VSRC) with an output resistance (RSRC). The voltage on the

A terminal is sensed by Pin 1 of the ADA4530-1 in a noninverting

gain configuration (or a unity-gain configuration). The B terminal is

driven to a suitable reference voltage (signal ground in this case).

If all error sources are ignored, the output of the circuit is as follows:

VOUT=VSRC1+RFRS

(2)

Figure 105. Voltage Buffer Circuit

The TIA circuit models the current output sensor as a current

source (ISRC) with a shunt resistance (RSRC). The current from the

A terminal is connected to the inverting input pin of the ADA4530-1

and the feedback resistor (RF). The B terminal and the noninverting

input of the amplifier are driven to a suitable reference voltage

(signal ground in this case). The negative feedback of the circuit

suppresses any voltage changes at the A terminal. This suppres-

sion is accomplished by forcing all current through the feedback

resistor.

If all error sources are ignored, the output of the circuit is as follows:

VOUT = ISRC × RF

(3)

Figure 106. TIA Circuit

INPUT BIAS CURRENT

The input bias current of the amplifier is a major error source in high

impedance electrometer circuits.

Like other semiconductor amplifiers, the input bias current of the

ADA4530-1 has an exponential dependence on temperature. The

input bias current of the ADA4530-1 increases by a factor of 2.5

for every 10°C increase in temperature. Refer to the input bias

current vs. temperature graphs (see Figure 34 to Figure 36) for

typical temperature performance. Notice that the exponential diode

currents cease to be the dominant contributor to the input bias

current at temperatures below 60°C to 70°C. The residual 100 aA

to 200 aA (aA = 10−18 A) bias currents are dominated by other

leakage paths that are highly sensitive to environmental conditions.

These vanishingly small bias currents require highly controlled

laboratory conditions to measure. Most practical applications are

dominated by other errors, and the ADA4530-1 input bias current

can be considered to be zero for temperatures less than 70°C. The

input bias current of the ADA4530-1 can only be guaranteed to ±20

fA due to the measurement limitations of a production environment,

even though the achievable input bias currents are more than an

order of magnitude lower.

The input bias current affects the buffer circuit by loading down

the voltage sensor. The input bias current is forced to flow through

the output resistance of the sensor, which creates an error voltage,

VERR.

VERR = IB+(RSRC)

(4)

The magnitude of this voltage error can be significant with very high

impedance sensors operating at high temperature. For example,

the input bias current can generate a maximum voltage error of 25

mV from a 100 GΩ sensor operating at 125°C.

The input bias current affects the TIA circuit by summing together

with the sensor current. Both of these currents flow through the

feedback resistor to generate the output voltage as follows:

VOUT = (ISRC + IB−)RF

(5)

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

current can be resolved accurately. For example, if the acceptable

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