ADA4530-1ARZ-R7 datasheet(33/51 Pages) AD

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ADA4530-1

Data Sheet

Rev. A | Page 32 of 50

HIGH IMPEDANCE MEASUREMENTS

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.

Measurements 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 TIA. Buffer circuits are useful for measuring voltage output

sensors with high output resistance. Some example sensors include

pH probes and reference electrodes (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 chambers.

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 errors sources are ignored, the output of the circuit is as

follows:

















SRC

OUT

ADA4530-1

RSRC

VSRC

IB+

RIN

VOS

VOUT

VOLTAGE

SENSOR

RSHUNT

+IN

OUT

–IN

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 suppression 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 = ISRCRF

ADA4530-1

RSRC

ISRC

IB–

RIN

RSHUNT

VOS

VOUT

CURRENT

SENSOR

V+

IRF

Figure 106. Transimpedance Amplifier 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 exponen-

tial diode currents cease to be the dominate 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 = IB+(RSRC)

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

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