ADA4530-1 Datasheet, PDF (33/51 Pages) Analog Devices – Femtoampere Input Bias Current Electrometer Amplifier

ADA4530-1
Data Sheet
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:
V_OUT
 V_{SRC }1
R_F
R_S

VOLTAGE
SENSOR
R_SRC
V_SRC
A
R_SHUNT
1 ^+IN
R_{IN IB+}
–IN
8
V_OS
ADA4530-1
OUT
6
V_OUT
R_F
current through the feedback resistor. If all error sources are
ignored, the output of the circuit is as follows:
VOUT = ISRCRF
V+
CURRENT
SENSOR
R_SHUNT
A
I_SRC
R_SRC
I_RF
R_F
8
R_{IN IB–}
ADA4530-1
6
1
B
V_OS
V_OUT
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.
R_S
B
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
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
Rev. A | Page 32 of 50