Datasheet AD202, AD204 (Analog Devices) - 8

AD202/AD204 Using Isolated Power.
Both the AD202 and the AD204 provide
Operation at Reduced Signal Swing.
Although the nominal ±7.5 V power outputs referenced to input common. These may be output signal swing for the AD202 and AD204 is ± 5 V, there used to power various accessory circuits that must operate at may be cases where a smaller signal range is desirable. When the input common-mode level; the input zero adjustment pots that is done, the fixed errors (principally offset terms and output described above are an example, and several other possible uses noise) become a larger fraction of the signal, but nonlinearity is are shown in the section titled Application Examples. reduced. This is shown in Figure 15. The isolated power output of the AD202 (400 mA total from
0.025
either or both outputs) is much more limited in current capacity than that of the AD204, but it is sufficient for operating micro- power op amps, low power references (such as the AD589),
0.020
adjustment circuits, and the like. The AD204 gets its power from an external clock driver, and
0.015
can handle loads on its isolated supply outputs of 2 mA for each supply terminal (+7.5 V and –7.5 V) or 3 mA for a single loaded output. Whenever the external load on either supply is more
0.010
than about 200 mA, a 1 mF tantalum capacitor should be used to bypass each loaded supply pin to input common.
NONLINEARITY – % span 0.005
Up to 32 AD204s can be driven from a single AD246 (or equi- valent) clock driver when the isolated power outputs of the AD204s are loaded with less than 200 mA each, at a worst-case
0 0 1 2 3 4 5
supply voltage of 14.25 V at the clock driver. The number of
OUTPUT SIGNAL SWING – V
AD204s that can be driven by one clock driver is reduced by Figure 15. Nonlinearity vs. Signal Swing one AD204 per 3.5 mA of isolated power load current at 7.5 V, distributed in any way over the AD204s being supplied by that
PCB Layout for Multichannel Applications.
The pinout of clock driver. Thus a load of 1.75 mA from +V the AD204Y has been designed to make very dense packing ISO to –VISO would also count as one isolator because it spans 15 V. possible in multichannel applications. Figure 16a shows the recommended printed circuit board (PCB) layout for the simple It is possible to increase clock fanout by increasing supply volt- voltage-follower connection. When gain-setting resistors are age above the 14.25 V minimum required for 32 loads. One present, 0.25" channel centers can still be achieved, as shown in additional isolator (or 3.5 mA unit load) can be driven for each Figure 16b. 40 mV of increase in supply voltage up to 15 V. Therefore if the minimum supply voltage can be held to 15 V – 1%, it is possible
CHANNEL INPUTS
to operate 32 AD204s and 52 mA of 7.5 V loads. Figure 14
0 1 2
shows the allowable combinations of load current and channel count for various supply voltages.
50 TOTAL I = 0mA ISO 40 0.1” TOTAL GRID I = 35mA ISO 30 TOTAL = 80mA IISO AL TOT I = 70mA ISO 20 CLK COM NUMBER OF AD204s DRIVEN 10 OPERATION IN THIS REGION EXCEEDS CLK 4mA LOAD LIMIT PER AD204 0 OUT COM 14.25 14.50 14.75 15.0 MINIMUM SUPPLY VOLTAGE
Figure 14. AD246 Fanout Rules
CHANNEL OUTPUTS TO MUX
Figure 16a. (NOTE: Circuit figures shown on this page are for SIP-style packages. Refer to Page 3 for proper DIP package pinout.) –8– REV. D Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION PRODUCT HIGHLIGHTS SPECIFICATIONS AD246–SPECIFICATIONS AD246 Pin Designations PIN DESIGNATIONS ORDERING GUIDE DIFFERENCES BETWEEN THE AD202 AND AD204 INSIDE THE AD202 AND AD204 USING THE AD202 AND AD204 Powering the AD204 AD246 Clock Drive Input Configurations Adjustments Common-Mode Performance Dynamics and Noise Using Isolated Power Operation at Reduced Signal Swing PCB Layout for Multichannel Applications Synchronization APPLICATIONS EXAMPLES Low Level Sensor Inputs Process Current Input with Offset High Compliance Current Source Motor Control Isolator. Floating Current Source/Ohmmeter Photodiode Amplifier OUTLINE DIMENSIONS Revision History