TYPICAL PERFORMANCE CURVES
(CONT)
T
A
= +25°C; V
S1
= V
S2
=
±15V;
and R
L
= 2kΩ, unless otherwise noted.
STEP RESPONSE
+10
0
STEP RESPONSE
+10
0
Output Voltage (V)
Output Voltage (V)
–10
–10
+10
0
–10
+10
0
–10
0
250
Time (µs)
500
0
50
Time (µs)
100
THEORY OF OPERATION
The ISO120 and ISO121 isolation amplifiers comprise input
and output sections galvanically isolated by matched 1pF
capacitors built into the ceramic barrier. The input is duty-
cycle modulated and transmitted digitally across the barrier.
The output section receives the modulated signal, converts it
back to an analog voltage and removes the ripple component
inherent in the demodulation. The input and output sections
are laser-trimmed for exceptional matching of circuitry com-
mon to both input and output sections.
FREE-RUNNING MODE
An input amplifier (A1, Figure1) integrates the difference
between the input current (V
IN
/200kΩ) and a switched
±100µA
current source. This current source is implemented
by a switchable 200µA source and a fixed 100µA current
sink. To understand the basic operation of the input section,
assume that V
IN
= 0. The integrator will ramp in one
direction until the comparator threshold is exceeded. The
comparator and sense amp will force the current source to
switch; the resultant signal is a triangular waveform with a
50% duty cycle. If V
IN
changes, the duty cycle of the
integrator will change to keep the average DC value at the
output of A1 near zero volts. This action converts the input
voltage to a duty-cycle modulated triangular waveform at
the output of A1 near zero volts. This action converts the
input voltage to a duty-cycle modulated triangular wave-
form at the output of A1 with a frequency determined by the
internal 150pF capacitor. The comparator generates a fast
rise time square wave that is simultaneously fed back to keep
A1 in charge balance and also across the barrier to a
differential sense amplifier with high common-mode rejec-
tion characteristics. The sense amplifier drives a switched
current source surrounding A2. The output stage balances
the duty-cycle modulated current against the feedback cur-
rent through the 200kΩ feedback resistor, resulting in an
average value at the Sense pin equal to V
IN
. The sample and
hold amplifiers in the output feedback loop serve to remove
undesired ripple voltages inherent in the demodulation process.
SYNCHRONIZED MODE
A unique feature of the ISO120 and ISO121 is the ability to
synchronize the modulator to an external signal source. This
capability is useful in eliminating trouble-some beat fre-
quencies in multi-channel systems and in rejecting AC
signals and their harmonics. To use this feature, external
capacitors are connected at C
1
and C
2
(Figure 1) to change
the free-running carrier frequency. An external signal is
applied to the Ext Osc pin. This signal forces the current
source to switch at the frequency of the external signal. If
V
IN
is zero, and the external source has a 50% duty cycle,
operation proceeds as described above, except that the switch-
ing frequency is that of the external source. If the external
signal has a duty cycle other than 50%, its average value is
not zero. At start-up, the current source does not switch until
the integrator establishes an output equal to the average DC
value of the external signal. At this point, the external signal
is able to trigger the current source, producing a triangular
waveform, symmetrical about the new DC value, at the
output of A1. For V
IN
= 0, this waveform has a 50% duty
cycle. As V
IN
varies, the waveform retains its DC offset, but
varies in duty cycle to maintain charge balance around A1.
Operation of the demodulator is the same as outlined above.
Synchronizing to a Sine
or Triangle Wave External Clock
The ideal external clock signal for the ISO120/121 is a
±4V
sine wave or
±4V,
50% duty-cycle triangle wave. The
ext osc
pin of the ISO120/121 can be driven directly with a
±3V
to
±5V
sine or 25% to 75% duty-cycle triangle wave and the ISO
amp's internal modulator/demodulator circuitry will synchro-
nize to the signal.
Synchronizing to signals below 400kHz requires the addition
of two external capacitors to the ISO120/121. Connect one
capacitor in parallel with the internal modulator capacitor and
connect the other capacitor in parallel with the internal de-
modulator capacitor as shown in Figure 1.
ISO120/121
6
BURR-BROWN [ BURR-BROWN CORPORATION ]
