2. Demodulator loadings should be closely matched so their
output voltages will be equal. (Unequal demodulator
output voltages will produce a gain error.) At the 2MΩ
level, a matching error of 5% will cause an additional
gain error of 0.25%.
ages. These two features of 3656 provide a great deal of
versatility in possible isolation and power supply hook-ups.
When external supplies are applied, the rectifying diodes
(D1 through D4) are reverse biased and the internal voltage
sources are decoupled from the amplifiers (see Figure 1).
Note that when external supplies are used, they must never
be lower than the internal supply voltage.
3. Voltage swings at demodulator outputs should be limited
to 5V. The output may be distorted if this limit is
exceeded. This constrains the maximum allowed gains of
the input and output stages. Note that the voltage swings
at demodulator outputs are tested with 2MΩ load for a
minimum of 5V.
Three-Port
The power supply connections in Figure 2 show the full
three-port isolation configuration. The system has three
separate grounds with no galvanic connections between
them. The two external 0.47µF capacitors at pins 12 and 16
filter the rectified isolated voltage at the output stage. Filter-
ing on the input stage is provided by internal capacitors. In
this configuration continuous isolation voltage ratings are:
3500V between pins 3 and 17; 3500V between pins 3 and
19; 1000V between pins 17 and 19.
4. Total current drawn from the internal isolated supplies
must be limited to less than ±5mA per supply and limited
to a total of 10mA. In other words, the combination of
external and internal current drawn from the internal
circuitry which feeds the +V, –V, V+ and V– pins should
be limited to 5mA per supply (total current to +V, –V, V+
and V– limited to 10mA). The internal filter capacitors
for ±V are 0.01µF. If more than 0.1mA is drawn to
provide isolated power for external circuitry (see Figure
12), additional capacitors are required to provide ad-
equate filtering. A minimum of 0.1µF/mA is recom-
mended.
Two-Port Bipolar Supply
Figure 3 shows two-port isolation which uses an external
bipolar supply with its common connected to the output
stage ground (pin 17). One of the supplies (either + or –
could be used) provides power to the pulse generator (pins
19 and 20). The same sort of configuration is possible with
the external supplies connected to the input stage. With the
connection shown, filtering at pins 12 and 16 is not required.
In this configuration continuous isolation voltage rating is:
3500VDC between pins 3 and 17; not applicable between
pins 17 and 19; 3500VDC between pins 3 and 19.
5. The input voltage at pin 7 (noninverting input to A1) must
not exceed the voltage at pin 4 (negative supply voltage
for A1) in order to prevent a possible lockup condition. A
low leakage diode connected between pins 7 and 4, as
shown in Figure 2, can be used to limit this input voltage
swing.
6. Impedances seen by each amplifier’s + and – input
terminals should be matched to minimize offset voltages
caused by amplifier input bias currents. Since the de-
modulators have a 100kΩ output resistance, the amplifier
input not connected to the demodulator should also see
100kΩ.
Two-Port Single Supply
Figure 4 demonstrates two-port isolation using a single
polarity supply connected to the output common (pin 17).
The other polarity of supply for A2 is internally generated
(thus the filtering at pin 12). This isolated power configura-
tion could be used at the input stage as well and either
polarity of supply could be employed. In this configuration
continuous isolation voltage rating is: 3500V between pins
3 and 17; 3500V between pins 3 and 19; not applicable
between pins 17 and 19.
7. All external filter capacitors should be mounted as close
to the respective supply pins as possible in order to
prevent excessive ripple voltages on the supplies or at the
output. (Optimum spacing is less than 0.5”. Ceramic
capacitors recommended.)
SIGNAL CONFIGURATIONS
Unity Gain Noninverting
POWER AND
SIGNAL CONFIGURATIONS
NOTE: Figures 2, 3 and 4 are used to illustrate both signal
and power connection configurations. In the circuits shown,
the power and signal configurations are independent so that
any power configuration could be used with any signal
configuration.
The signal path portion of Figure 2 shows the 3656 is its
simplest gain configuration: unity gain noninverting. The
two 100kΩ resistors provide balanced resistances to the
inverting and noninverting inputs of the amplifiers. The
diode prevents latch up in case the input voltage goes more
negative than the voltage at pin 4.
Noninverting With Gain
The signal path portion of Figure 3 demonstrates two addi-
tional gain configurations: gain in the output stage and
noninverting gain in the input stage. The following equa-
tions apply:
ISOLATED POWER CONFIGURATIONS
The 3656 is designed with isolation between the input, the
output, and the power connections. The internally generated
isolated voltages supplied to A1 and A2 may be overridden
with external voltages greater than the internal supply volt-
®
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3656
BB [ BURR-BROWN CORPORATION ]
