ISO121 datasheet(6/18 Pages) TI2

6 / 18 page

ISO120/121

STEP RESPONSE

Time (µs)

100

+10

–10

+10

–10

STEP RESPONSE

Time (µs)

250

500

+10

–10

+10

–10

TYPICAL PERFORMANCE CURVES (CONT)

A = +25°C; VS1 = VS2 = ±15V; and RL = 2kΩ, unless otherwise noted.

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 200

µA source and a fixed 100µA current

sink. To understand the basic operation of the input section,

assume that V

= 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

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 200k

Ω 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 C2 (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

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.

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