ISO124

7

SBOS074C

www.ti.com

1

2

7

9

10

8

PGA102

ISO124

ISO150

1

2

3

16

4

6

7

8

5

15

15

+15V –15V

+15V –15V

V

IN

V

OUT

A

0

A

1

above 250kHz, the behavior is similar to that of a sampling

amplifier. The typical characteristic “Signal Response to

Inputs Greater Than 250kHz” shows this behavior graphi-

cally; at input frequencies above 250kHz, the device gener-

ates an output signal component of reduced magnitude at a

frequency below 250kHz. This is the aliasing effect of sam-

pling at frequencies less than 2 times the signal frequency

(the Nyquist frequency). Note that at the carrier frequency

and its harmonics, both the frequency and amplitude of the

aliasing go to zero.

ISOLATION MODE VOLTAGE INDUCED ERRORS

IMV can induce errors at the output as indicated by the plots of IMV

vs Frequency. It should be noted that if the IMV frequency exceeds

250kHz, the output also will display spurious outputs (aliasing) in

response will be identical to that shown in the “Signal Response to

Inputs Greater Than 250kHz” typical characteristic. This occurs

because IMV-induced errors behave like input-referred error sig-

nals. To predict the total error, divide the isolation voltage by the

IMR shown in the “IMR versus Frequency” typical performance

curve and compute the amplifier response to this input-referred

error signal from the data given in the “Signal Response to Inputs

Greater Than 250kHz” typical characteristic. For example, if a

800kHz 1000Vrms IMR is present, then a total of

[(–60dB) + (–30dB)] x (1000V) = 32mV error signal at 200kHz plus

a 1V, 800kHz error signal will be present at the output.

HIGH IMV dV/dt ERRORS

As the IMV frequency increases and the dV/dt exceeds

1000V/

µs, the sense amp may start to false trigger, and the

output will display spurious errors. The common-mode cur-

rent being sent across the barrier by the high slew rate is the

cause of the false triggering of the sense amplifier. Lowering

the power-supply voltages below

±15V may decrease the

dV/dt to 500V/

µs for typical performance.

HIGH VOLTAGE TESTING

Texas Instruments has adopted a partial discharge test

criterion that conforms to the German VDE0884 Optocoupler

Standards. This method requires the measurement of minute

current pulses (< 5pC) while applying 2400Vrms, 60Hz high-

voltage stress across every ISO124 isolation barrier. No

partial discharge may be initiated to pass this test. This

criterion confirms transient overvoltage (1.6 x 1500Vrms)

protection without damage to the ISO124. Lifetest results

verify the absence of failure under continuous rated voltage

and maximum temperature.

This new test method represents the “state-of-the art” for

non-destructive high-voltage reliability testing. It is based on

the effects of non-uniform fields that exist in heterogeneous

dielectric material during barrier degradation. In the case of

void non-uniformities, electric field stress begins to ionize the

void region before bridging the entire high-voltage barrier.

The transient conduction of charge during and after the

ionization can be detected externally as a burst of 0.01-0.1

µs

current pulses that repeat on each ac voltage cycle. The

minimum ac barrier voltage that initiates partial discharge is

defined as the “inception voltage.” Decreasing the barrier

voltage to a lower level is required before partial discharge

ceases and is defined as the “extinction voltage.” We have

characterized and developed the package insulation pro-

cesses to yield an inception voltage in excess of 2400Vrms

so that transient overvoltages below this level will not dam-

age the ISO124. The extinction voltage is above 1500Vrms

so that even overvoltage induced partial discharge will cease

once the barrier voltage is reduced to the 1500Vrms (rated)

level. Older high-voltage test methods relied on applying a

large enough overvoltage (above rating) to break down

marginal parts, but not so high as to damage good ones. Our

new partial discharge testing gives us more confidence in

barrier reliability than breakdown/no breakdown criteria.

FIGURE 3. Programmable-Gain Isolation Channel with Gains

of 1, 10, and 100.

FIGURE 2. Basic Signal and Power Connections.

+V

S1

+V

S2

Gnd

Gnd

V

IN

V

OUT

–V

S1

±V

S1

±V

S2

–V

S2

1

µF1µF 1µF

1

µF

Isolation Barrier

ISO124