R

1

C

2

1000pF

A

1

OPA602

4.75kW

7

R

2

9.76kW

Gnd1

Gnd2

C

1

220pF

15

16

V

IN

–V

S1

2

+V

S1

1

–V

S2

10

+V

S2

9

8

ISO

122

In

Gnd

V

ISO122

SBOS160A – NOVEMBER 1993 – REVISED JANUARY 2015

www.ti.com

Application Information (continued)

package insulation processes have been characterized and developed to yield an inception voltage in excess of

2400 Vrms so that transient overvoltages below this level will not damage the ISO122. The extinction voltage is

above 1500 Vrms so that even overvoltage induced partial discharge will cease once the barrier voltage is

reduced to the 1500 Vrms (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.

9.2 Typical Application

9.2.1 Output Filter

Figure 10. ISO122 With Output Filter for Improved Ripple

9.2.1.1 Design Requirements

The ISO122 isolation amplifiers (ISO amps) have a small (10 to 20 mVp-p typical) residual demodulator ripple at

the output. A simple filter can be added to eliminate the output ripple without decreasing the 50-kHz signal

bandwidth of the ISO amp.

9.2.1.2 Detailed Design Procedure

The ISO122 is designed to have a 50-kHz single-pole (Butterworth) signal response. By cascading the ISO amp

with a simple 50-kHz, Q = 1, two-pole, low-pass filter, the overall signal response becomes three-pole

Butterworth. The result is a maximally flat 50-kHz magnitude response and the output ripple reduced below the

noise level. Figure 10 shows the complete circuit. The two-pole filter is a unity-gain Sallen-Key type consisting of

A1, R1, R2, C1, and C2. The values shown give Q = 1 and f–3-dB bandwidth = 50 kHz. Because the op amp is

connected as a unity-gain follower, gain and gain accuracy of the ISO amp are unaffected. Using a precision op

amp such as the OPA602 also preserves the DC accuracy of the ISO amp.

10

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