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ISO122 Datasheet(PDF) 6 Page - Burr-Brown (TI) |
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ISO122 Datasheet(HTML) 6 Page - Burr-Brown (TI) |
6 / 13 page 6 ® ISO122 THEORY OF OPERATION The ISO122 isolation amplifier uses an input and an output section galvanically isolated by matched 1pF isolating ca- pacitors built into the plastic package. 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. Input and output sections are fabricated, then laser trimmed for exceptional circuitry match- ing common to both input and output sections. The sections are then mounted on opposite ends of the package with the isolating capacitors mounted between the two sections. The transistor count of the ISO122 is 250 transistors. MODULATOR An input amplifier (A1, Figure 1) 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 modulator, assume that V IN = 0.0V. 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. The internal oscillator forces the current source to switch at 500kHz. The resultant capacitor drive is a complementary duty-cycle modulation square wave. DEMODULATOR The sense amplifier detects the signal transitions across the capacitive barrier and drives a switched current source into integrator A2. The output stage balances the duty-cycle modulated current against the feedback current through the 200k Ω feedback resistor, resulting in an average value at the V OUT pin equal to VIN. The sample and hold amplifiers in the output feedback loop serve to remove undesired ripple voltages inherent in the demodulation process. BASIC OPERATION SIGNAL AND SUPPLY CONNECTIONS Each power supply pin should be bypassed with 1 µF tanta- lum capacitors located as close to the amplifier as possible. The internal frequency of the modulator/demodulator is set at 500kHz by an internal oscillator. Therefore, if it is desired to minimize any feedthrough noise (beat frequencies) from a DC/DC converter, use a π filter on the supplies (see Figure 4). ISO122 output has a 500kHz ripple of 20mV, which can be removed with a simple two pole low-pass filter with a 100kHz cutoff using a low cost op amp. See Figure 4. The input to the modulator is a current (set by the 200k Ω integrator input resistor) that makes it possible to have an input voltage greater than the input supplies, as long as the output supply is at least ±15V. It is therefore possible when using an unregulated DC/DC converter to minimize PSR related output errors with ±5V voltage regulators on the isolated side and still get the full ±10V input and output swing. An example of this application is shown in Figure 10. CARRIER FREQUENCY CONSIDERATIONS The ISO122 amplifier transmits the signal across the isola- tion barrier by a 500kHz duty cycle modulation technique. For input signals having frequencies below 250kHz, this system works like any linear amplifier. But for frequencies above 250kHz, the behavior is similar to that of a sampling amplifier. The signal response to inputs greater than 250kHz 1pF 200 µA 100 µA 200k Ω Isolation Barrier 1pF 1pF 200 µA 100 µA 200k Ω V IN + – Osc Gnd 1 –V S1 +V S1 V OUT S/H G = 6 S/H G = 1 Gnd 2 +V S2 –V S2 – + A2 Sense Sense FIGURE 1. Block Diagram. A1 150pF 150pF 1pF || |
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