AD8212
Rev. B | Page 10 of 16
HIGH VOLTAGE OPERATION USING AN EXTERNAL
PNP TRANSISTOR
In this mode of operation, the supply current (I
BIAS
) of the
AD8212 circuit increases based on the supply range and the
R
BIAS
resistor chosen. For example
The AD8212 offers features that simplify measuring current in
the presence of common-mode voltages greater than 65 V. This
is achieved by connecting an external PNP transistor at the
output of the AD8212, as shown in Figure 23. The V
CE
break-
down voltage of this PNP becomes the operating common-mode
range of the AD8212. PNP transistors with breakdown voltages
exceeding 300 V are inexpensive and readily available in small
packages.
if
V+ = 500 V and R
BIAS
= 500 kΩ
I
BIAS
= (V+ − 5 V)/R
BIAS
then,
I
BIAS
= (500 – 5)/500 kΩ = 990 μA
In high voltage operation, it is recommended that I
BIAS
remain
within 200 μA to 1 mA. This ensures that the bias circuit is
turned on, allowing the device to function as expected. At the
same time, the current through the bias circuit/regulator is
limited to 1 mA. Refer to Figure 19 and Figure 21 for I
BIAS
and
V+ information when using the AD8212 in a high voltage
configuration.
R
OUT
Q2
AD8212
B
TTE
Y
R
SHUNT
05942-004
OUTPUT
CURRENT
COMPENSATION
BIAS
CIRCUIT
LOAD
R1 R2
A1
Q1
VOUT
R
BIAS
8
6325
1
When operating the AD8212, as depicted in Figure 23,
Transistor Q2 can be a FET or a bipolar PNP transistor. The
latter is much less expensive, however the magnitude of I
OUT
conducted to the output resistor (R
OUT
) is reduced by the
amount of current lost through the base of the PNP. This leads
to an error in the output voltage reading.
The AD8212 includes an integrated patented circuit, which
compensates for the output current that is lost through the base
of the external PNP transistor. This ensures that the correct
transconductance of the amplifier is maintained. The user can
opt for an inexpensive bipolar PNP, instead of a FET, while
maintaining a comparable level of accuracy.
OUTPUT CURRENT COMPENSATION CIRCUIT
The base of the external PNP, Q2, is connected to ALPHA
(Pin 6) of the AD8212. The current flowing in this path is
mirrored inside the current compensation circuit. This
current then flows in Resistor R2, which is the same value
as Resistor R1. The voltage created by this current across
Resistor R2, displaces the noninverting input of Amplifier A1
by the corresponding voltage. Amplifier A1 responds by driving
the base of Transistor Q1 so as to force a similar voltage
displacement across Resistor R1, thereby increasing I
OUT
.
Figure 23. High Voltage Operation Using External PNP
The AD8212 features an integrated 5 V series regulator. This
regulator ensures that at all times COM (Pin 2), which is the
most negative of all the terminals, is always 5 V less than the
supply voltage (V+). Assuming a battery voltage (V+) of 100 V,
it follows that the voltage at COM (Pin 2) is
(V+) – 5 V = 95 V
The base emitter junction of Transistor Q2, in addition to the
V
be
of one internal transistor, makes the collector of Transistor Q1
approximately equal to
Because the current generated by the output compensation
circuit is equal to the base current of Transistor Q2, and the
resulting displacements across Resistor R1 and Resistor R2 result
in equal currents, the increment of current added to the output
current is equivalent to the base current of Transistor Q2.
Therefore, the integrated output current compensation circuit
has corrected I
OUT
such that no error results from the base
current lost at Transistor Q2.
95 V + 2(V
be(Q2)
) = 95 V + 1.2 V = 96.2 V
This voltage appears across external Transistor Q2. The voltage
across Transistor Q1 is
100 V – 96.2 V = 3.8 V
In this manner, Transistor Q2 withstands 95.6 V and the
internal Transistor Q1 is only subjected to voltages well below
its breakdown capability.
This feature of the AD8212 greatly improves I
OUT
accuracy and
allows the user to choose an inexpensive bipolar PNP (with low
beta) with which to monitor current in the presence of high
voltages (typically several hundred volts).
