ISO121 datasheet(7/18 Pages) TI2

7 / 18 page

ISO120/121

−3dB ≈

1. 2

200 k

Ω 150 pF + C

()

FIGURE 1. Block Diagram.

1, C2 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

0.01

µF

5kHz to 10kHz

0.022

µF

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 1k

Ω

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 • C1. A larger value for

2 will decrease bandwidth and improve stability:

Where:

–3dB ≈ –3dB bandwidth of ISO amp with external C2 (Hz)

2 = External demodulator capacitor (f)

For example, with C

2 = 0.01µF, the f–3dB bandwidth of the

ISO120/121 is approximately 600Hz.

1pF

S/H

G = 1

S/H

G = 6

Sense

OUT

Signal

Com 2

Gnd 2 –V

Gnd 1 –V

Signal

Com 1

(1)

200k

Ω

150pF

100µA

Sense

200µA

100µA

Sense

200µA

200k

Ω

150pF

(1)

Isolation Barrier

NOTE: (1) Optional. See text.

30k

Ω

16k

Ω

Ext

Osc

16k

Ω

50pF

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