NCN5192NGEVB
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13
is a virtual ground, as this assumption is only valid for DC
signals. We can, however, find a relationship between input
amplitude and output amplitude. We know that the positive
amplifier input voltage has the following form, due to the
summing network:
V
)
+
ǒ
R
1
ńń
R
2
ńń
ǒ
R
4
)
R
5
Ǔ
Ǔ
ǒ
V
in
R
1
*
V
out
R
2
Ǔ
The amplifier is configured as an integrator for low
frequencies, but for high frequencies, the amplifier
configuration has a gain of 1, and the transistor is configured
as a voltage follower, so we can conclude that for AC
frequencies V
+
= V
out
. Taking this into account, we get the
following equation:
V
out
+
R
2
R
1
ǒ
R
1
ńń
R
2
ńń
ǒ
R
4
)
R
5
Ǔ
Ǔ
ǒ
R
1
ńń
R
2
ńń
ǒ
R
4
)
R
5
Ǔ
Ǔ
)
R
2
V
in
Reconfiguring for the unknown R
1
:
R
1
+
R
2
ǒ
R
4
)
R
5
Ǔ
V
in
)
V
out
ǒ
R
2
)
R
3
Ǔ
V
out
ǒ
1
)
ǒ
R
4
)
R
5
Ǔ
Ǔ
The amplifier is configured as an integrator for low
frequencies. Care must be taken that the 3-dB frequency of
the integrator is below the HART band, so that the amplifier
gain in that band is independent of frequency. The resistor
R
3
is chosen so that it compensates for input bias current.
This is achieved by taking a value close to the resistance seen
on the positive terminal. This means that the capacitor C
2
needs to be chosen so that (2
p
R
3
C
2
)
−
1
< 1 kHz.
Figure 18. Sample Slave Implementation
Master Implementation
An example of a possible master implementation is shown
in Figure 19.
The current loop master has a sense resistor over which the
current flowing through the loop can be measured. The value
of this resistor varies depending on the sensitivity required
and range of the ADC. A HART Master can have a sense
resistor ranging from 230
W
to 600
W
. Increasing the sense
resistor will result in higher amplitude HART signal
received, but will also reduce the voltage available on the
slave side. Furthermore, if you wish to sense the analog
transmitted signal, the MSB of your DAC may limit the
resistor size. If this limitation is too stringent, the sense
resistor can be split in two resistors, as shown in the figure,
effectively creating a resistor divider.
To transmit a HART signal, the TxA signal will need to be
amplified, as the NCN5192 transmit circuit can only drive
high impedance circuits (>30 k
W
). An additional
operational amplifier is required. Depending on the sense
resistor used, some gain or attenuation may be required to
get a 1 mA peak-to-peak HART output signal. This can be
accomplished by the resistors R
3
and R
4
. For a typical sense
resistor of 500
W
, a unity gain suffices and a unity gain
operational amplifier configuration can be used instead.
The amplifier however has a low impedance output,
which cannot be paralleled with the sense resistor, as this
would cause problems when the slave is transmitting. This
problem is solved by adding a series switch (such as
MC74VHC1G66DTT1G), controlled by the RTS signal.
For a normally open switch, the nRTS signal as applied to the
NCN5192 must be inverted first. To reduce power usage, the
operational amplifier can be disabled when the transmitter
is turned off. This is both done by inserting PNP transistor
Q
1
on the V
DD
connection of the amplifier.
To couple the signal into the current loop, a single
capacitor was used. For other coupling techniques see
application note AND8346/D.
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