ADA4530-1 Datasheet, PDF (35/51 Pages) Analog Devices – Femtoampere Input Bias Current Electrometer Amplifier

ADA4530-1
Data Sheet
insulation resistance values in glass epoxy (such as FR-4) PCB
materials. Resistance values of 10 TΩ to 100 TΩ are achievable.
A 10 TΩ insulation resistance creates a 1% error with the 100 GΩ
sensor used in previous examples. Insulation resistance does not
have an exponential temperature dependence like the amplifier
errors previously discussed in the Input Bias Current section and
the Input Resistance section, which makes insulation resistance
the dominate error source at lower temperatures (less than 70°C).
The effect on the insulation resistance on the TIA circuit depends
on the leakage path. The insulation resistance between the A
terminal and B terminal of the current sensor affects the circuit
in the same way as the amplifier input resistance. This error is
extremely small because the voltage across the insulation is
equal to the offset voltage of the amplifier. A much more
significant error is created from insulation paths to conductors
with significantly different potentials. This type of leakage path
is shown as lumped element, RSHUNT, in the TIA circuit (see
Figure 106). In this example, the leakage path is created from
the positive supply voltage (V+) to the A terminal. If the
positive supply voltage is 5 V relative to signal ground, 500 fA
flows through the insulation resistance of 10 TΩ. This large
error dominates the amplifier input bias current and input
resistance errors over the entire temperature range.
Leakage paths to high voltages can also affect the buffer circuit
with equally ruinous results.
GUARDING
High source impedances and low error requirements can create
insulation resistance requirements that are unrealistically high.
Fortunately, a technique called guarding can reduce these
requirements to a reasonable level. The concept of guarding is
to surround the high impedance conductor with another
conductor (guard) that is driven to the same voltage potential. If
there is no voltage across the insulation resistance (between
high impedance conductor and guard), there cannot be any
current flowing through it.
The ADA4530-1 uses guarding techniques internally, and it has
a very high performance guard buffer integrated. The output of
this buffer is made available externally to simplify the
implementation of guarding at the circuit level.
The voltage buffer circuit (see Figure 105) has been modified to
show the implementation of the guard (see Figure 107). In this
model, a conductor (VGRD) is added, and it completely separates
the high impedance (A) node from the low impedance (B) node
at a different voltage. The insulation resistance is modeled as
two resistances: all of the insulation between the A conductor
and the guard conductor (RSHUNT1), and all of the insulation
between the guard conductor and the B conductor (RSHUNT2).
The ADA4530-1 guard buffer then drives this guard conductor
(through Pin 2 and Pin 7) to the A terminal voltage. If the A
node and VGRD node are exactly the same voltage, no current
flows through the insulation resistance, RSHUNT1.
In practice, the voltage across RSHUNT1 cannot be 0 V, the guard
buffer offset voltage contributes to the difference in voltage
potential between the A node and VGRD node. For the
ADA4530-1, this offset voltage is trimmed to provide offsets
less than 100 μV when the input common-mode voltage is 1.5 V
from the supply rails. The guard buffer offset voltage and drift
are specified in Table 1, Table 2, and Table 3.
For example, assume that the voltage sensor produces an output
of 1 V. Without guarding, the 10 TΩ insulation resistance
creates an error current of 100 fA. With the guard, the voltage
across the insulation resistance is limited to 100 μV. The guard
limits the error current to 0.01 fA. In this example, the guard
reduces the error by a factor of 10⁴ to an insignificant level.
VOLTAGE
SENSOR
A
ADA4530-1
1
R_SRC
V_SRC
R_SHUNT1
2
V_{GRD 7}
8
R_SHUNT2
6
V_OUT
R_F
R_S
B
Figure 107. Voltage Buffer Circuit with Guard
DIELECTRIC RELAXATION
Dielectric relaxation (also known as dielectric absorption or
soakage) is a property of all insulating materials that can limit
the performance of electrometer circuits that need to settle to a
few femtoamperes.
Dielectric relaxation is the delay in polarization of the dielectric
molecules in response to a changing electric field. This delay is
a property of all insulating materials. The magnitude and the
time constant of the delay depend on the specific dielectric
material. The delays in some materials can be minutes or even
hours.
Dielectric relaxation is a problem for electrometer circuits
because small displacement currents flow through the insulator
in response to the polarization of the molecules. Delays in
polarization cause delays in the dissipation of these currents,
which can dominate the settling time in these circuits.
In the context of capacitors, dielectric relaxation is called
dielectric absorption. Capacitors are specified with a test that
measures the residual open-circuit voltage after a specific
charge/discharge cycle. For electrometer circuits, it is more
useful to consider the short-circuit currents produced from step
changes in a test voltage.
A simple lumped circuit model of an insulator is connected to
the test voltage source (see Figure 108). The majority of the
dielectric polarizes instantly; this is modeled as Capacitor C1. A
small percentage of the dielectric polarizes slowly with a time
constant of τ2, modeled as Capacitor C2 and Resistor R2.
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