Datasheet

7
ISO120/121
f
−3dB
≈
1. 2
200kΩ 150 pF + C
2
()
FIGURE 1. Block Diagram.
C
1
, C
2
ISO120/121
EXTERNAL CLOCK MODULATOR, DEMODULATOR
FREQUENCY RANGE EXTERNAL CAPACITOR
400kHz to 700kHz none
200kHz to 400kHz 500pF
100kHz to 200kHz 1000pF
50kHz to 100kHz 2200pF
20kHz to 50kHz 4700pF
10kHz to 20kHz 0.01µF
5kHz to 10kHz 0.022µF
The value of the external modulator capacitor, C
1
, depends on
the frequency of the external clock signal. Table I lists
recommended values.
Synchronizing to a 400kHz to 700kHz
Square-Wave External Clock
At frequencies above 400kHz, an internal clamp and filter
provides signal conditioning so that a square-wave signal can
be used to directly drive the ISO120/121. A square-wave
external clock signal can be used to directly drive the ISO120/
121 ext osc pin if: the signal is in the 400kHz to 700kHz
frequency range with a 25% to 75% duty cycle, and ±3V to
±20V level. Details of the internal clamp and filter circuitry
are shown in Figure 1.
Synchronizing to a 10% to 90%
Duty-cycle External Clock
With the addition of the signal conditioning circuit shown in
Figure 2, any 10% to 90% duty-cycle square-wave signal can
be used to drive the ISO120/121 ext osc pin. With the values
shown, the circuit can be driven by a 4Vp-p TTL signal. For
a higher or lower voltage input, increase or decrease the 1kΩ
resistor, R
X
, proportionally. e.g. for a ±4V square wave
(8Vp-p) R
X
should be increased to 2kΩ.
The value of C
X
used in the Figure 2 circuit depends on the
frequency of the external clock signal. Table II shows recom-
mended capacitor values.
Note: For external clock frequencies below 400kHz, external
modulator/demodulator capacitors are required on the
ISO120/121 as before.
TABLE I. Recommended ISO120/121 External Modulator/
Demodulator Capacitor Values vs External Clock
Frequency.
The value of the external demodulator capacitor, C
2
, depends
on the value of the external modulator capacitor. To assure
stability, C
2
must be greater than 0.8 • C
1
. A larger value for
C
2
will decrease bandwidth and improve stability:
Where:
f
–3dB
≈ –3dB bandwidth of ISO amp with external C
2
(Hz)
C
2
= External demodulator capacitor (f)
For example, with C
2
= 0.01µF, the f
–3dB
bandwidth of the
ISO120/121 is approximately 600Hz.
1pF
1pF
1pF
1pF
S/H
G = 1
S/H
G = 6
Sense
V
OUT
Signal
Com 2
Gnd 2 –V
S2
+V
S2
Gnd 1 –V
S1
+V
S1
Signal
Com 1
V
IN
C
1H
C
1L
C
1
(1)
200kΩ
150pF
A1
100µA
Sense
200µA
X
100µA
Sense
200µA
200kΩ
150pF
C
2L
C
2H
C
2
(1)
X
A2
Isolation Barrier
NOTE: (1) Optional. See text.
30kΩ 16kΩ
Ext
Osc
16kΩ 50pF