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ISO121BG Datasheet(PDF) 9 Page - Burr-Brown (TI) |
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ISO121BG Datasheet(HTML) 9 Page - Burr-Brown (TI) |
9 / 16 page 9 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 2 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. +V S1 or +VS2 Signal Com 1 or Signal Com 2 –V S1 or –VS2 100k Ω 1M 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 f 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. C 1 Ext Osc on ISO120 (pin 22) C 2 10k Ω 6 5 8 2 3 TTL f IN 2.5k Ω 200 Ω +15V +5V f IN 140E-6 () C 1 = – 350pF C 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 R 2 V IN V OUT R 1 Sense R 2 R 1 200k Gain = 1 + + R 1 ( ) R 1 || R2 GND1 1k Ω 2k Ω R 1 R 2 V IN V OUT GND1 Sense |
Similar Part No. - ISO121BG |
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Similar Description - ISO121BG |
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