Datasheet AD210 (Analog Devices) - 7

AD210 Isolated Power:
The AD210 provides isolated power at the The isolated power supplies exhibit some ripple which varies as input and output ports. This power is useful for various signal a function of load. Figure 16a shows this relationship. The conditioning tasks. Both ports are rated at a nominal ± 15 V at AD210 has internal bypass capacitance to reduce the ripple to a 5 mA. point where performance is not affected, even under full load. The load characteristics of the isolated power supplies are Since the internal circuitry is more sensitive to noise on the shown in Figure 15. For example, when measuring the load negative supplies, these supplies have been filtered more heavily. rejection of the input isolated supplies V Should a specific application require more bypassing on the iso- ISS, the load is placed between +V lated power supplies, there is no problem with adding external ISS and –VISS. The curves labeled VISS and VOSS are the individual load rejection characteristics of the input and the capacitors. Figure 16b depicts supply ripple as a function of output supplies, respectively. external bypass capacitance under full load. There is also some effect on either isolated supply when loading
1V
the other supply. The curve labeled CROSSLOAD indicates the sensitivity of either the input or output supplies as a function of the load on the opposite supply.
100mV 30 CROSSLOAD 10mV
( )
+VISS +VOSS RIPPLE – Peak-Peak Volts
( )
–VISS 25 –V VOLTAGE OSS 1mV VOSS 0.1µF 1µF 10µF 100µF V SIMULTANEOUS OSS CAPACITANCE VISS
Figure 16b. Isolated Power Supply Ripple vs. Bypass
V SIMULTANEOUS ISS
Capacitance (Volts p-p, 1 MHz Bandwidth, 5 mA Load)
20 0 5 10 APPLICATIONS EXAMPLES CURRENT – mA Noise Reduction in Data Acquisition Systems:
Transformer Figure 15. Isolated Power Supplies vs. Load coupled isolation amplifiers must have a carrier to pass both ac and dc signals through their signal transformers. Therefore, Lastly, the curves labeled VOSS simultaneous and VISS simulta- some carrier ripple is inevitably passed through to the isolator neous indicate the load characteristics of the isolated power sup- output. As the bandwidth of the isolator is increased more of the plies when an equal load is placed on both supplies. carrier signal will be present at the output. In most cases, the The AD210 provides short circuit protection for its isolated ripple at the AD210’s output will be insignificant when com- power supplies. When either the input supplies or the output pared to the measured signal. However, in some applications, supplies are shorted to input common or output common, particularly when a fast analog-to-digital converter is used fol- respectively, no damage will be incurred, even under continuous lowing the isolator, it may be desirable to add filtering; other- application of the short. However, the AD210 may be damaged wise ripple may cause inaccurate measurements. Figure 17 if the input and output supplies are shorted simultaneously. shows a circuit that will limit the isolator’s bandwidth, thereby reducing the carrier ripple.
+V +V OSS ISS 100 16 R AD542 R 1 17 –V V V OSS OUT SIG 75 +VOSS 19 AD210 0.001µF 0.002µF 2 18 –V 50 ISS –V +V RIPPLE – mV p-p OSS 14 ISS +VOSS 3
R (kΩ) = ( )
112.5 25 fC (kHz) 15 –VISS –V 4 OSS 30 29 0 0 1 2 3 4 5 6 7 +15V LOAD – mA
Figure 17. 2-Pole, Output Filter Figure 16a. Isolated Supply Ripple vs. Load
Self-Powered Current Source
(External 4.7 µF Bypass) The output circuit shown in Figure 18 can be used to create a Under any circumstances, care should be taken to ensure that self-powered output current source using the AD210. The 2 kΩ the power supplies do not accidentally become shorted. resistor converts the voltage output of the AD210 to an equiva- –6– REV. A