ISO121 datasheet(9/18 Pages) TI2

9 / 18 page

ISO120/121

output are not significant under these circumstances unless

the input signal contains significant components above

250kHz.

There are two ways to use these characteristics. One is to

move the carrier frequency low enough that the troublesome

signal components are attenuated to an acceptable level as

shown in Signal Response vs Carrier Frequency. This in

effect limits the bandwidth of the amplifier. The Synchroni-

zation Range performance curve shows the relationship

between carrier frequency and the value of C

1. To maintain

stability, C

2 must also be connected and must be equal to or

larger in value than C

1. C2 may be further increased in value

for additional attenuation of the undesired signal compo-

nents and provides the additional benefit of reducing the

residual carrier ripple at the output. See the Bandwidth vs C

performance curve.

When periodic noise from external sources such as system

clocks and DC/DC converters are a problem, ISO120 and

ISO121 can be used to reject this noise. The amplifier can be

synchronized to an external frequency source, f

EXT, placing

the amplifier response curve at one of the frequency and

amplitude nulls indicated in the Signal Response vs Carrier

Frequency performance curve. For proper synchronization,

choose C

1 as shown in the Synchronization Range perfor-

mance curve. Remember that C

2 ≥ C1 is a necessary condi-

tion for stability of the isolation amplifier. This curve shows

the range of lock at the fundamental frequency for a 4V

sinusoidal signal source. The applications section shows the

ISO120 and ISO121 synchronized to isolation power sup-

plies, while Figure 6 shows circuitry with opto-isolation

suitable for driving the Ext Osc input from TTL levels.

FIGURE 4a. Gain Adjust.

FIGURE 4b. Gain Setting.

S1 or +VS2

Signal Com 1

Signal Com 2

–V

S1 or –VS2

100k

Ω

10k

Ω

FIGURE 5. V

OS Adjust.

resolution with a typical trim potentiometer. The output will

have some sensitivity to power supply variations. For a

±100mV trim, power supply sensitivity is 8mV/V at the

output.

CARRIER FREQUENCY CONSIDERATIONS

As previously discussed, the ISO120 and ISO121 amplifiers

transmit the signal across the iso-barrier by a duty-cycle

modulation technique. This system works like any linear

amplifier for input signals having frequencies below one

half the carrier frequency, f

C. For signal frequencies above

C/2, the behavior becomes more complex. The Signal Re-

sponse vs Carrier Frequency performance curve describes

this behavior graphically. The upper curve illustrates the

response for input signals varying from DC to f

C/2. At input

frequencies at or above f

C/2, the device generates an output

signal component that varies in both amplitude and fre-

quency, as shown by the lower curve. The lower horizontal

scale shows the periodic variation in the frequency of the

output component. Note that at the carrier frequency and its

harmonics, both the frequency and amplitude of the re-

sponse go the zero. These characteristics can be exploited in

certain applications. It should be noted that when C

1 is zero,

the carrier frequency is nominally 500kHz and the –3dB

point of the amplifier is 60kHz. Spurious signals at the

FIGURE 6. Synchronization with Isolated Drive Circuit for

Ext Osc Pin.

Ext Osc on

ISO120 (pin 22)

10k

Ω

TTL

2.5k

Ω

200

Ω

+15V

+5V

140E-6

()

1 =

– 350pF

2 = 10 X C1, with a minimum 10nF

2.5k

Ω

6N136

ISOLATION MODE VOLTAGE

Isolation mode voltage (IMV) is the voltage appearing be-

tween isolated grounds Gnd 1 and Gnd 2. IMV can induce

error at the output as indicated by the plots of IMV vs

Frequency. It should be noted that if the IMV frequency

exceeds f

C/2, the output will display spurious outputs in a

manner similar to that described above, and the amplifier

response will be identical to that shown in the Signal Re-

sponse vs Carrier Frequency performance curve. This occurs

OUT

Sense

200k

Gain = 1 +

(

)

1 || R2

GND1

Ω

OUT

GND1

Sense

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