AN-1373: ADA4530-1 Femtoampere Level Input Bias Current Measurement (Rev. 0) PDF

AN-1373
APPLICATION NOTE
One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com
ADA4530-1 Femtoampere Level Input Bias Current Measurement
INTRODUCTION
The ADA4530-1 is a single, electrometer grade operational
amplifier with a femtoampere (10−15) level input bias current
(IB) and an ultralow offset voltage. Its ultralow input bias currents
are production tested at 25°C and 125°C to ensure that the device
meets its performance goals in a system application. Figure 1
and Figure 2 show the outstanding input bias current performance
of the device over temperature and input common-mode voltage.
1000
–40°C TO +125°C LIMIT
VSY = 10V
VCM = VSY/2
RH < 10%
100
Low input bias current amplifiers are also typically available in
T0-99 packages. These packages allow users to air wire the high
impedance input pins or to use Teflon® insulator standoffs to
prevent leakage current. These techniques increase manufacturing
costs and are incompatible with modern automated PCB assembly
processes. The surface-mounted, plastic package provided by the
ADA4530-1 bypasses this legacy assembly approach and is reliable
in a modern surface-mount manufacturing environment.
–40°C TO +85°C LIMIT
IB (fA)
10
1
0.1
IB+
IB–
0.001
10
20
30
40
50
60
70
80
90 100 110 120 130
TEMPERATURE (°C)
13419-101
0.01
0
The ADA4530-1 has a unique pinout. The input and supply pins
are placed on opposite sides of the package to prevent leakage. For
ease of user design, the ADA4530-1 features an integrated guard
buffer. The guard buffer drives the guard ring surrounding the
input pins, thus minimizing both input pin leakage in a printed
circuit board (PCB) design and board component count. The guard
buffer output pins are also strategically placed next to the input pins
to enable easy routing of the guard ring. For more information on
guarding and physical implementation of guarding techniques,
refer to the ADA4530-1 data sheet.
This application note highlights several different methods for
measuring the ADA4530-1 femtoampere level input bias current
feature in the SOIC package using the ADA4530-1R-EBZ-TIA
or the ADA4530-1R-EBZ-BUF evaluation board.
Figure 1. Input Bias Current (IB) vs. Temperature
300
VSY = 10V
27 CHANNELS
TA = 125°C
200
PREFERRED
COMMON-MODE RANGE
100
IB+ (fA)
0
–100
–200
–300
13419-203
–400
–600
0
1
2
3
4
5
VCM (V)
6
7
8
9
10
13419-102
–500
Figure 2. Noninverting Input Bias Current (IB+) vs. Common-Mode Voltage (VCM)
The ADA4530-1 operates over the −40°C to +125°C industrial
temperature range and is available in an 8-lead SOIC package. It is
well suited for applications that require very low input bias current
and low offset voltage, such as preamplifier for a wide variety of
current output transducers (photodiodes, photomultiplier tubes),
spectrometry, chromatography, and high impedance buffering for
chemical sensors.
Figure 3. Photograph of the ADA4530-1R-EBZ-TIA
Figure 3 shows a photograph of the ADA4530-1R-EBZ-TIA.
Note that hereafter, ADA4530-1R-EBZ refers to both the
ADA4530-1R-EBZ-TIA and the ADA4530-1R-EBZ-BUF.
For full details on the ADA4530-1, see the ADA4530-1 data sheet.
The ADA4530-1R-EBZ user guide (UG-865) should also be
consulted in conjunction with this application note.
Rev. 0 | Page 1 of 11
AN-1373
Application Note
TABLE OF CONTENTS
Introduction ...................................................................................... 1
Revision History ............................................................................... 2
Measuring Total Input Capacitance with an Input Test
Capacitor ....................................................................................6
Cleaning and Handling .................................................................... 3
Measuring IB+ with Known Input Capacitance .....................6
Measurement Techniques ................................................................ 4
ΔVOUT (Two Test Output Voltage) Measurement......................8
Keithley 6430 Measurement........................................................ 4
IB− Measurement ........................................................................8
Capacitive Integration Measurement......................................... 5
IB+ Measurement ..................................................................... 10
Measuring Total Input Capacitance with an Input Series
Resistor ...................................................................................... 6
Conclusion....................................................................................... 11
REVISION HISTORY
10/15—Revision 0: Initial Version
Rev. 0 | Page 2 of 11
Application Note
AN-1373
CLEANING AND HANDLING
The input bias current measurement methods outlined in this
application note use the ADA4530-1R-EBZ. Properly clean the
ADA4530-1R-EBZ before each measurement to remove any
contaminants, such as solder flux, saline moisture, dirt, or dust.
This cleaning maintains the low leakage performance of the
ADA4530-1R-EBZ.
4.
An effective cleaning procedure follows:
7.
1.
2.
3.
Soak the ADA4530-1R-EBZ in an ultrasonic bath with
cleanroom grade isopropyl alcohol for 15 minutes. Ultrasonic
cleaning uses ultrasound at a high frequency, which creates
cavitation in the cleaning solution. This process helps to
remove contaminants on the surface of the ADA4530-1R-EBZ
and in the areas under the soldered components that are hard
to reach. The next cleaning steps require the use of fresh
isopropyl alcohol.
Remove the ADA4530-1R-EBZ from the ultrasonic bath
with a pair of forceps. Rinse and flush the ADA4530-1R-EBZ
with isopropyl alcohol to remove any contaminant residue.
Flood the ADA4530-1R-EBZ with isopropyl alcohol and
gently scrub it with an acid brush. Concentrate on the areas
between the U1 pins, the input traces to J1, the guard ring,
and the area within SHIELD1.
5.
6.
8.
9.
Rinse and flush the ADA4530-1R-EBZ again with isopropyl
alcohol.
Repeat Step 3 and Step 4 for the bottom of the ADA45301R-EBZ.
Give a final flush of the top and the bottom of the ADA45301R-EBZ with isopropyl alcohol.
Use compressed dry air to dry the ADA4530-1R-EBZ. Blow
air around the U1 pins, the input traces to J1, and the guard
ring area. Direct the compressed air under J1 and U1 as well.
Bake the ADA4530-1R-EBZ in the temperature chamber at
125°C for 15 minutes to ensure that the ADA4530-1R-EBZ
is completely dry.
After cleaning, place the covers on the metal shields. The
metal shields also help to prevent any contact to the
guarded area.
Always handle the ADA4530-1R-EBZ by the edges and never
touch the area within SHIELD1 or SHIELD2.
Rev. 0 | Page 3 of 11
AN-1373
Application Note
Metal shields on the ADA4530-1R-EBZ prevent capacitive
coupling from external interference. The shields also prevent
contact to the guarded area and therefore prevent contamination
from fingerprints or dust to the high impedance inputs. The
ADA4530-1R-EBZ-BUF and the ADA4530-1R-EBZ-TIA each
have two 1.0 in × 1.5 in × 0.25 in metal shields preassembled on
the top (SHIELD1) and bottom (SHIELD3) of the board. In
addition, a larger 1.5 in × 3 in × 0.75 in metal shield (SHIELD2)
is provided separately with each evaluation board. Metal clips
are assembled on each board to hold SHIELD2 in place. The
guard buffer drives these shields to the DUT noninverting pin
potential.
To provide an even more robust electrostatic shielding, shield
the ADA4530-1R-EBZ with a metal box when evaluating the
input bias current. This box within a box construction is
effective because the outer shield is driven to ground and the
inner shield is driven with guard. For more information on
guarding and shielding, refer to the ADA4530-1 data sheet.
The ADA4530-1R-EBZ is also preassembled with a 499 Ω output
resistor, which isolates any output load from the amplifier output
and prevents oscillation from excessive capacitive loading.
499Ω
KEITHLEY
6430
Figure 4. Simplified Schematic for IB+ Measurement with the Keithley 6430
SourceMeter
Figure 5. Keithley 6430 SourceMeter
The procedures for measurement test setup follow:
1.
2.
3.
4.
Throughout this application note, IB+ refers to the input bias
current flowing through the DUT noninverting pin and IB−
refers to the input bias current flowing through the DUT
inverting pin. IB refers to both IB+ and IB−.
The next three sections describe the different input bias current
measurement methods that can be implemented on the
ADA4530-1R-EBZ.
KEITHLEY 6430 MEASUREMENT
ADA4530-1
VOUT
TO
MULTIMETER
13419-005
The ADA4530-1R-EBZ is available in two default configurations:
the ADA4530-1R-EBZ-BUF, which is the device under test (DUT)
in buffer mode, and the ADA4530-1R-EBZ-TIA, which is the DUT
in transimpedance mode.
13419-004
MEASUREMENT TECHNIQUES
5.
6.
The typical input bias current of the ADA4530-1 at 25°C is
<1 fA, and this ultralow current is impossible to measure
accurately with any available meter. For example, the offset
current of a Keithley 6430 SourceMeter® at a 1 pA range is
already limited to 7 fA. Therefore, to measure IB, it is necessary
to heat the DUT to increase the input bias current to a measurable
value. At 125°C, IB is measurable with a maximum of ±250 fA,
per the data sheet specifications.
Use the ADA4530-1R-EBZ-BUF. Ensure that SHIELD1 and
SHIELD3 are closed.
Place the ADA4530-1R-EBZ-BUF in a metal box (see
Figure 6).
Cover the metal box and place the ADA4530-1R-EBZ-BUF
in a temperature chamber.
Connect the input triax connector (J1) to the IN/OUT port of
the Keithley 6430 Remote PreAmp via a triax cable that can
withstand up to 125°C. Many triax cables are only functional
up to 85°C. A 125°C triax cable can be built using Harbour
Industries M17/131-RG403 TRX cable and a Model 5218
triax connector from Pomona Electronics.
Set the temperature chamber to 125°C. Do not place the
Keithley 6430 Remote PreAmp or SourceMeter in the
temperature chamber.
After the temperature chamber has reached 125°C, allow
the evaluation board to soak for an hour.
13419-007
For the purpose of IB+ testing, the ADA4530-1R-EBZ-BUF is
placed in a temperature chamber. To measure IB+, connect the
ADA4530-1 noninverting pin (through J1) directly to the Keithley
6430 SourceMeter with a triax cable. For more information
about triax cabling, refer to the ADA4530-1 data sheet.
Figure 6. Metal Box (Connected to Signal Ground) Enclosing the
ADA4530-1R-EBZ-BUF
See Figure 4 for a simplified schematic and Figure 5 for the
SourceMeter used.
Rev. 0 | Page 4 of 11
Application Note
AN-1373
1.
2.
3.
Provide supplies to the ADA4530-1R-EBZ-BUF:
a. V+ (J3): 5 V
b. GND (J4): 0 V
c. V− (J5): −5 V
Provide the test setup for the Keithley 6430 SourceMeter:
a. Auto range: off
b. Source voltage (V): 0 V
c. Measure current (I): set the current range to 1 pA
d. Number of power line cycles (NPLC): 1 (normal speed)
e. Filter: auto
Log the measurement data for 300 seconds at 1 sample per
second (SPS) and calculate the average IB+. Note that longer
measurement periods yield more accurate averaging. The
average IB+ should be less than 100 fA.
Figure 7 shows data collected from the measurement of one
typical DUT for 5000 seconds. The average measured IB+ is
approximately 11 fA to 12 fA at 125°C. Typical input bias current
at 125°C varies from device to device. Expect most DUTs to
measure less than 100 fA with the ADA4530-1R-EBZ-BUF.
20
VSY = ±5V
VCM = 0V
TA = 125°C
18
16
To calculate IB+, use the following equation:
I B+ =
C P dVOUT
dt
where:
CP is the input capacitance.
dVOUT/dt is the change in amplifier output voltage over time.
Figure 8 shows a simplified schematic of the capacitive
integration method.
TEST BOX
CP
499Ω
ADA4530-1
VOUT
TO
MULTIMETER
13419-011
Test procedures for measuring IB+ for the ADA4530-1 using the
Keithley 6430 SourceMeter follow:
Figure 8. Simplified Schematic of the Capacitive Integration Method
As with the measurement method described in the Keithley 6430
Measurement section, place the ADA4530-1R-EBZ-BUF in a
temperature chamber. Access the noninverting input of the
DUT using a triax cable/connector connected to a test box
outside of the temperature chamber (see Figure 9). The test box
consists of a triax connector with access to the noninverting input
signal, guard, and signal ground potential (see Figure 10). It
provides the means to short the noninverting input pin of the
DUT to ground outside the temperature chamber.
IB+ (fA)
14
12
10
13419-106
8
6
ONE SAMPLE IS TAKEN
EVERY SECOND WITH
THE KEITHLEY 6430 AFTER
THE UNIT HAS BEEN BAKED
AT 125°C FOR AN HOUR.
2
0
0
500
1000 1500 2000 2500 3000 3500 4000 4500 5000
TIME (Seconds)
Figure 9. Capacitive Integration Measurement Method
13419-006
4
INPUT
Figure 7. Noninverting Input Bias Current (IB+) vs. Time
GUARD
CAPACITIVE INTEGRATION MEASUREMENT
C dVC
IC =
dt
This relationship can be used to calculate IB+ based on the rate
at which it charges the DUT input capacitance. Observe this
charging of the input capacitance by measuring the output of
the DUT in buffer configuration with the noninverting input
unconnected.
GND
13419-103
Current flowing through a capacitor (IC) can be calculated using
the capacitance value (C) and the measured change in output
voltage across the capacitor over time (dVC/dt) with the
following equation:
Figure 10. Test Box and Triax Connector
To calculate input bias current from the output voltage ramp
rate of the DUT, the input capacitance value must be known.
Therefore, the total input capacitance (CP), which consists of the
capacitance of the DUT, the board, the traces, and the triax cable/
connector, must be measured. However, the input capacitance is
very low and difficult to measure. Two recommended methods
for cross checking the value of CP are using an input series
resistor and using an input test capacitor.
Rev. 0 | Page 5 of 11
AN-1373
Application Note
Measuring Total Input Capacitance with an Input Series
Resistor
Measurement guidelines follow:
1.
2.
3.
4.
5.
6.
7.
8.
Use the ADA4530-1R-EBZ-BUF. Short the triax guard to
the amplifier guard on the ADA4530-1R-EBZ-BUF using JP3.
Place the ADA4530-1R-EBZ-BUF in a metal box. Connect the
ADA4530-1R-EBZ-BUF to the test box with a triax cable/
connector (see Figure 9).
Connect a function generator to the input pin of the test
box through an input series resistor, RS, and an oscilloscope
to the output of the ADA4530-1R-EBZ-BUF (see Figure 11).
Short the input series resistor, RS.
Using the function generator, apply a 1 kHz, 1 V p-p input
sine wave.
Use the oscilloscope to check that VOUT = VIN.
Remove the short across RS. An 8 MΩ resistor is used for
RS. To reduce stray capacitance, use multiple resistors in
series instead of one large resistor. In this case, four 2 MΩ
resistors are soldered in series.
Slowly increase the input signal frequency until the output
drops by a factor of √2. At this frequency, the input signal
frequency at the −3 dB point is
f −3dB =
1
2 × π × RS × CP
1
2 × π × RS × f −3dB
Hence,
CP =
ADA4530-1
TEST BOX
VIN
TO
FUNCTION
GENERATOR
ADA4530-1
VOUT
TO
OSCILLOSCOPE
On average, the two input capacitance measurement methods yield
a measured input parasitic capacitance of about 2 pF for the
ADA4530-1R-EBZ-BUF. Note that this capacitance value is
less than the amplifier specification on the data sheet and low
considering the length of triax cable used. The internal guard buffer
bootstraps most of the physical capacitances, which greatly reduces
the effective input capacitance value.
3.
4.
This method calculates the total input capacitance by measuring
the attenuation of an input voltage using a voltage divider
created from the input capacitance and a known capacitance.
5.
Measurement guidelines follow:
3.
CP
499Ω
Measuring IB+ with Known Input Capacitance
2.
Measuring Total Input Capacitance with an Input Test
Capacitor
2.
CTEST
Figure 12. Measuring Total Input Capacitance with an
Input Test Capacitor
1.
Figure 11. Measuring Total Input Capacitance with an
Input Series Resistor
1.
CTEST (VIN − VOUT )
VOUT
The procedures for test measurement setup follow:
VOUT
TO
OSCILLOSCOPE
13419-009
499Ω
RS
CP
6.
See Figure 8 for the simplified schematic of this capacitive
integration method.
TEST BOX
VIN
TO
FUNCTION
GENERATOR
5.
With the known input capacitance, the output voltage ramp rate of
the DUT can be measured to calculate IB+.
Hence,
CP =
4.
13419-010
This method calculates the total input capacitance by measuring
the frequency of the pole created from the interaction between a
known resistance and the input capacitance.
the input pin of the test box through CTEST and an oscilloscope
to the output of the ADA4530-1R-EBZ-BUF. See Figure 12.
Using a function generator, apply a 1 kHz, 1 V p-p input
sine wave.
Using the oscilloscope, measure the peak-to-peak output
voltage.
The input/output transfer function is as follows:
VOUT
CTEST
=
VIN
CP + CTEST
Use the ADA4530-1R-EBZ-BUF. Short the triax guard to the
amplifier guard on the ADA4530-1R-EBZ-BUF using JP3.
Place the ADA4530-1R-EBZ-BUF in a metal box. Connect the
ADA4530-1R-EBZ-BUF to the test box with a triax cable/
connector (See Figure 9).
Place a capacitor with a known value, CTEST, at the input pin. A
10 pF test capacitor is used. Connect a function generator to
6.
7.
8.
Rev. 0 | Page 6 of 11
Use the ADA4530-1R-EBZ-BUF. Ensure that SHIELD1
and SHIELD3 are closed.
Short the triax guard to the amplifier guard on the
ADA4530-1R-EBZ-BUF using JP3.
Place the ADA4530-1R-EBZ-BUF in a metal box (see Figure 9).
Place the metal box containing the ADA4530-1R-EBZ-BUF in
the temperature chamber.
Connect the input triax connector (J1) to the test box (see
Figure 8). Use a triax cable that can withstand up to 125°C.
Many triax cables are only functional up to 85°C. A 125°C
triax cable can be built using the Harbour Industries
M17/131-RG403 TRX cable and the Model 5218 triax
connector from Pomona Electronics.
Connect the output of the board to a multimeter via the output
BNC connector or the output terminal block. In the lab, a
5.5 digit to 6.5 digit multimeter that supports data logging
was used.
Set the temperature chamber to 125°C.
After the temperature chamber has reached 125°C, allow
the ADA4530-1R-EBZ-BUF to soak for an hour.
Application Note
AN-1373
Test procedures for measuring the IB+ of the ADA4530-1 with
the capacitive integration measurement method follow:
1.
2.
3.
4.
5.
6.
Provide the following supplies to the ADA4530-1R-EBZ-BUF:
a. V+ (J3): 5 V
b. GND (J4): 0 V
c. V− (J5): −5 V
Provide the test setup for the Keithley 2000:
a. Auto range: off
b. Custom range: 10 V
c. Time delay: 1 msec
d. Measurement delay: 1 sec
e. NPLC: 1 (normal speed)
Using the test box, short the DUT noninverting input to
signal ground for approximately 10 seconds.
Remove the short to ground and leave the noninverting pin
unconnected.
Capacitive current (ICAP) flowing across the input capacitance
appears as a change in the DUT output voltage.
Log the output voltage with the multimeter for 300 seconds
at 1 SPS.
Figure 13 and Figure 14 show the board output voltage vs. time
and the capacitive current vs. time, respectively.
VSY = ±5V
4.0 TA = 125°C
OUTPUT VOLTAGE (V)
3.5
3.0
2.5
2.0
1.5
1.0
0.5
Note that at lower temperatures, the ADA4530-1 has an ultralow
level of input bias current and that dielectric relaxation is an
additional error limiting the accuracy of this method. These
dielectric relaxation currents can be larger than the actual bias
currents. Therefore, measurement using this method is performed
at the elevated temperature of 125°C where the input bias current
dominates over dielectric effects. Refer to the ADA4530-1 data
sheet for more details on dielectric relaxation.
0
100
200
300
400
500
600
700
800
900
1000
TIME (Seconds)
13419-012
–0.5
0
Figure 13. Output Voltage vs. Time
35
VSY = ±5V
TA = 125°C
IB+ = 25fA
With the capacitive integration method, the input bias current at
125°C measures less than 100 fA.
25
20
15
IB+
10
VOUT
TO
MULTIMETER
ADA4530-1
RP
5
0
–5
IRES CP
ICAP
Figure 15. Board Input Parasitic Capacitance Used to Measure Input Bias Current
0
100
200
300
400
500
600
700
800
TIME (Seconds)
900
1000
13419-013
–10
499Ω
13419-014
CAPACITIVE CURRENT, ICAP (fA)
30
Immediately after the noninverting input is left unconnected,
the input bias current (IB+) flows through the input parasitic
capacitance, causing a change in the output voltage. Measure the
capacitive current right after the noninverting pin is disconnected
from ground (see Figure 14) because during this short time period,
the capacitive current (ICAP) is a much closer value to IB+. As the
noninverting input voltage increases in magnitude, leakage current
through the parasitic resistance (IRES) on the board becomes
significant; therefore, the change of output voltage is a function
of both IRES and ICAP (see Figure 15).
In addition, the input bias current is not constant over the
input common-mode voltage range (see the input bias current
vs input common-mode voltage graphs in the ADA4530-1 data
sheet). This variation can account for the changing slope of
integration (see Figure 14). The output voltage stops ramping (see
Figure 13) when the input bias current equals the input commonmode voltage divided by RP. This condition tends to happen
outside the input common-mode voltage range when input
bias current is close to zero.
4.5
–1.0
See Figure 13. From 0 seconds to 10 seconds, the noninverting
input is shorted to ground. The output voltage thus measures close
to 0 V for the first 10 seconds. When the noninverting input is
disconnected from ground, the output voltage changes abruptly.
Because this is a manual disconnect, any vibration or charges
from the fingers can be coupled into the input and cause output
voltage spikes, as seen at time = 10 seconds. Relays are not used
to short and open the input pin because of their low insulation
resistance. A relay with low insulation resistance effectively
decreases the total resistance as seen at the noninverting input,
compromising measurement accuracy. Refer to the ADA4530-1
data sheet for more information on insulation resistance.
Figure 14. Capacitive Current vs. Time
Rev. 0 | Page 7 of 11
AN-1373
Application Note
The procedures for test measurement setup follow:
ΔVOUT (TWO TEST OUTPUT VOLTAGE)
MEASUREMENT
The first two measurement methods discussed in this application
note require heating the evaluation board to 125°C to increase
the input bias current to a measureable level. This ΔVOUT
measurement method allows user to measure IB+ (or IB−) at
room temperature without a temperature chamber.
This ΔVOUT method requires two tests to measure the input bias
current. The first test measures VOUT1, which corresponds to the
baseline offset voltage of the ADA4530-1R-EBZ. The second test
measures VOUT2, which is the sum of the baseline offset voltage and
the voltage drop created by IB− or IB+ flowing through the feedback
or input series resistor.
1.
2.
3.
4.
Step by step test procedures for measuring IB− follow:
1.
2.
Use the ADA4530-1R-EBZ-TIA for IB− measurement and the
ADA4530-1R-EBZ-BUF for IB+ measurement.
IB− Measurement
Figure 16 shows a schematic for measuring IB− using the ADA45301R-EBZ-TIA. The DUT is configured in a transimpedance
configuration, and the output voltage is measured and logged
using an 8.5 digit multimeter, the Keysight 3458A. Use a large
value feedback resistor on the order of gigaohms (GΩ) or
teraohms (TΩ) to obtain a measureable output voltage; for
example, the ADA4530-1 was characterized using a 1 TΩ (TΩ =
1012 Ω) resistor, which provides 1 mV per fA of sensitivity.
The ADA4530-1R-EBZ-TIA comes assembled with a 10 GΩ
SMT resistor at RF1. The feedback resistor converts IB− to an
output voltage (VOUT), where
3.
4.
5.
6.
7.
VOUT = IB− × Feedback Resistor
ADA4530-1
VOUT
TO
MULTIMETER
13419-015
RF1 OR RF2
499Ω
Figure 16. IB− Measurement Schematic
First, establish the baseline offset voltage of the ADA4530-1R-EBZTIA by shorting out the feedback resistor and measuring the
output voltage (VOUT1) of the DUT in a buffer configuration.
Then, remove the short and measure the output voltage with
the feedback resistor in place.
8.
Provide supplies to the ADA4530-1R-EBZ-TIA:
a. V+ (J3): +5 V
b. GND (J4): 0 V
c. V− (J5): −5 V
Provide the test set up for the Agilent 3458A.
a. Auto range: off
b. Manual range: 0.1 V
c. NPLC: 10
Measure the output voltage. Log the results and calculate the
average output voltage (VOUT1). Note that longer measurement
periods yield more accurate averaging. A test length of at
least 60 seconds accommodates for any warm up effects.
Turn off the supplies to the ADA4530-1R-EBZ-TIA.
Remove the short. By default, expect the amplifier to be in
a transimpedance configuration with a 10 GΩ feedback
resistor at RF1.
Place the cover on SHIELD1.
Measure the output voltage (a sum of the offset voltage of
the board and the voltage drop due to IB− flowing through
the feedback resistor). Log the results and calculate the
average output voltage (VOUT2). Suggested test length is 300
seconds. Averaging lowers the resistor noise. A 10 GΩ resistor
has about 12.8 μV/√Hz of thermal noise. With 300 seconds
of data, total integrated resistor noise is about 6 μV p-p.
This is equivalent to approximately 0.6 fA of measurement
inaccuracy. More averages provide diminishing returns due to
the increase in low frequency 1/f noise.
Calculate the input bias current by
I B 
9.
The feedback resistor converts IB− to a measureable output voltage.
VOUT2 is the sum of the baseline offset voltage and the voltage drop
created by IB− flowing through the feedback resistor.
Calculate IB− by
I B 
Use the ADA4530-1R-EBZ-TIA.
Remove the SHIELD1 cover. Short the feedback resistor
(RF1) using the P7 and VOUT pin sockets.
Place the ADA4530-1R-EBZ-TIA in a metal box.
Connect the output of the ADA4530-1R-EBZ-TIA to the
Keysight 3458A multimeter.
(VOUT2  VOUT1)
Feedback Resistor
VOUT2  VOUT1
10 GΩ
The equation in Step 8 assumes that the feedback resistance
value is accurately known. Verify this assumption by forcing a
known test current into the ADA4530-1R-EBZ-TIA and
measuring the output voltage. The Keithley 6430 SourceMeter
was used to source 250 pA into the inverting pin of the
DUT via J1. The output should read approximately
250 pA × 10 GΩ = 2.5 V
Any deviation in output voltage from the expected value
is due to the tolerance of the feedback resistor. The
preassembled 10 GΩ resistor, RF1, has a 10% tolerance.
Rev. 0 | Page 8 of 11
Application Note
AN-1373
2.0
Figure 17 and Figure 18 show VOUT1 and VOUT2 for a sample unit
over a 300 second interval and their associated average values.
Figure 19 shows the calculated IB−. Figure 20 shows the average
measured feedback resistance value, RF1. The resistance value is
within its 10% tolerance.
VOUT1 AVERAGE = 2.5µV
4.0
1.5
1.0
IB– (fA)
4.5
IB– AVERAGE = 0.2fA
3.5
0.5
0
–0.5
–1.0
2.5
2.0
–1.5
0
50
100
1.5
200
250
300
Figure 19. IB− vs Time
1.0
10.70
0
50
100
150
200
250
300
TIME (Seconds)
13419-017
0.5
0
150
TIME (Seconds)
13419-018
VOUT1 (µV)
3.0
RF1 AVERAGE = 10.6GΩ
10.65
RF1 (GΩ)
Figure 17. VOUT1 vs Time
20
VOUT2 AVERAGE = 4.3µV
10.60
15
VOUT2 (µV)
5
10.50
0
50
100
150
200
250
TIME (Seconds)
0
300
13419-019
10.55
10
Figure 20. Measured Feedback Resistance (RF1) vs Time
–10
0
50
100
150
200
TIME (Seconds)
Figure 18. VOUT2 vs Time
250
300
13419-117
–5
This measurement method can also be performed with larger
value feedback resistors. These large value resistors are often glass
encapsulated, hermetically sealed, and come in large footprints.
Using a higher value feedback resistor yields a better signal-tonoise ratio at the output and a more accurate measurement. The
ADA4530-1R-EBZ-TIA provides pin sockets that allow the use
of through-hole resistors (RF2) such as the Ohmite RX-1M
ultrahigh resistance, high stability, hermetically sealed resistor.
When using these large value through-hole resistors, remove RF1,
which is preassembled on the ADA4530-1R-EBZ-TIA. After the
rework, clean the ADA4530-1R-EBZ-TIA according to the
instructions in the Cleaning and Handling section before taking
any measurement. Insert the large through-hole resistor between
the P7 and VOUT pin sockets and measure IB− by repeating the
same steps outlined in this section. Change the magnitude of
current being sourced into the DUT in Step 9 accordingly.
Rev. 0 | Page 9 of 11
AN-1373
Application Note
IB+ Measurement
Use the ADA4530-1R-EBZ-BUF for IB+ measurement. This
measurement method is similar to the measurement method
outlined in the IB− Measurement section. See Figure 21 for the
IB+ measurement schematic. The input series resistor converts
IB+ to an output voltage (VOUT) at J2, where:
VOUT = IB+ × Input Series Resistor
RS1
OR
RS2
ADA4530-1
VOUT
TO
MULTIMETER
13419-020
499Ω
Establish the baseline offset voltage of the ADA4530-1R-EBZ-TIA
by shorting the DUT noninverting pin to ground. To do this,
connect a wire from the P7 to P4 (GND) pin sockets. The baseline
offset voltage is VOUT1. Remove the short and measure the output
voltage with the input series resistor in place. Users can assemble a
1206 or 1210 package size SMT resistor at RS1 or use a large
through-hole resistor from the P7 to P4 (GND) pin sockets. The
input series resistor converts the IB+ to a measureable output
voltage. VOUT2 is the sum of the baseline offset voltage and the
voltage drop created by IB+ flowing through the input series
resistor. Calculate IB+ by
I B+ =
Figure 21. IB+ Measurement Schematic
Rev. 0 | Page 10 of 11
VOUT2 − VOUT1
Input Series Resistor
Application Note
AN-1373
CONCLUSION
The ADA4530-1 is an electrometer grade amplifier with a
femtoampere level input bias current. This application note
details three methods for measuring the femtoampere level
input bias current: Keithley 6430 measurement, capacitive
integration measurement, and ΔVOUT (two test output voltage)
measurement.


The Keithley 6430 measurement method is a direct
measurement of IB+ using the Keithley 6430 SourceMeter.
This test method uses only a calibrated instrument for input
bias current measurement. However, because the DUT input
bias current is extremely low (<1 fA), it is impossible to
measure IB accurately at room temperature. The DUT must
be heated to increase IB to a measureable value. This method
thus requires a temperature chamber, a triax cable rated up to
125°C, and the Keithley 6430 SourceMeter. Therefore, the
setup cost is high in return for a simple direct measurement.
The capacitive integration method calculates IB by measuring
the change in amplifier output voltage over time with a
known input capacitance. This measurement can be
performed without a high-end source measurement unit
(SMU) and only requires a multimeter to monitor the
output voltage; however, the setup time is long. To calculate
the input bias current, the total input capacitance value
must be known. Therefore, the input capacitance must be
measured before the integration test. This measurement
method involves disconnecting wires by hand to avoid
using relays with low insulation resistance and requires a
temperature chamber to heat the DUT to increase IB to a
measurable level. In addition, IB is not constant over input
common-mode voltage. There is also only a short period of
time during which IB can be calculated from the capacitive
integration measurement method.

The ΔVOUT (two test output voltage) measurement method
requires two tests to measure input bias current. The first
test measures VOUT1, which corresponds to the baseline
offset voltage of the ADA4530-1R-EBZ. The second test
measures VOUT2, which is the sum of the baseline offset voltage
and the voltage drop created by IB− or IB+ flowing through
the feedback or input series resistor. The setup is fast and
the cost is low. The ADA4530-1R-EBZ-TIA is preassembled
with a 10 GΩ resistor, and measurement can be taken with
a multimeter. However, for a more accurate measurement, a
user can opt to use a larger value feedback or input series
resistor. These large value resistors are often glass encapsulated
and hermetically sealed, allowing them to achieve a high
degree of accuracy and stability. In return, their costs are
typically high. These resistors also require extraordinary
cleanliness and special cleaning procedures, and they must
be handled only by the terminals to avoid contamination.
For more information on the ADA4530-1, refer to the ADA4530-1
data sheet and the ADA4530-1R-EBZ user guide, UG-865.
©2015 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
AN13419-0-10/15(0)
Rev. 0 | Page 11 of 11
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