Datasheet AD202, AD204 (Analog Devices) - 7

AD202/AD204 180
Except at the highest useful gains, the noise seen at the output
G = 100
of the AD202 and AD204 will be almost entirely comprised of
G = 1 160
carrier ripple at multiples of 25 kHz. The ripple is typically
R
2 mV p-p near zero output and increases to about 7 mV p-p for
LO = 0 140
outputs of ± 5 V (1 MHz measurement bandwidth). Adding a capacitor across the output will reduce ripple at the expense of
120 RLO = 500
bandwidth: for example, 0.05 mF at the output of the AD204
RLO = 0
will result in 1.5 mV ripple at ± 5 V, but signal bandwidth will
CMR – dB 100 R
be down to 1 kHz.
LO = 10k 80 R
When the full isolator bandwidth is needed, the simple two-pole
LO = 10k
active filter shown in Figure 13 can be used. It will reduce ripple
60
to 0.1 mV p-p with no loss of signal bandwidth, and also serves as an output buffer.
4010 20 50 60 100 200 500 1k 2k 5k
An output buffer or filter may sometimes show output spikes
FREQUENCY – Hz
that do not appear at its input. This is usually due to clock noise Figure 10b. AD202 appearing at the op amp’s supply pins (since most op amps have
Dynamics and Noise.
Frequency response plots for the AD202 little or no supply rejection at high frequencies). Another com- and AD204 are given in Figure 11. Since neither isolator is slew- mon source of carrier-related noise is the sharing of a ground rate limited, the plots apply for both large and small signals. track by both the output circuit and the power input. Figure 13 Capacitive loads of up to 470 pF will not materially affect fre- shows how to avoid these problems: the clock/supply port of the quency response. When large signals beyond a few hundred Hz isolator does not share ground or 15 V tracks with any signal will be present, it is advisable to bypass –V circuits, and the op amp’s supply pins are bypassed to signal ISO and +VISO to IN COM with 1 mF tantalum capacitors even if the isolated supplies common (note that the grounded filter capacitor goes here as are not loaded. well). Ideally, the output signal LO lead and the supply com- mon meet where the isolator output is actually measured, e.g., At 50 Hz/60 Hz, phase shift through the AD202/AD204 is typically at an A/D converter input. If that point is more than a few feet 0.8∞ (lagging). Typical unit to unit variation is ±0.2∞ (lagging). from the isolator, it may be useful to bypass output LO to sup-
60
ply common at the isolator with a 0.1 mF capacitor.
AD204
In applications where more than a few AD204s are driven by a
AD202
single clock driver, substantial current spikes will flow in the
40
power return line and in whichever signal out lead returns to a low impedance point (usually output LO). Both of these tracks
AMPLITUDE 20
should be made large to minimize inductance and resistance;
RESPONSE – dB
ideally, output LO should be directly connected to a ground
I /V
plane which serves as measurement common.
OV 0 PHASE 0 RESPONSE
Current spikes can be greatly reduced by connecting a small
(G = 1)
inductance (68 mH–100 mH) in series with the clock pin of each
–20 –50
AD204. Molded chokes such as the Dale IM-2 series, with dc resistance of about 5 W, are suitable.
PHASE DEGREES –40 –100 10 20 50 100 200 500 1k 2k 5k 10k 20k FREQUENCY – Hz
Figure 11. Frequency Response at Several Gains
2200pF AD711
The step response of the AD204 for very fast input signals can
10k 10k
be improved by the use of an input filter, as shown in Figure 12.
POINT OF + + MEASUREMENT
The filter limits the bandwidth of the input (to about 5.3 kHz)
1000pF
so that the isolator does not see fast, out-of-band input terms
1.0 F 1.0 F
that can cause small amounts (±0.3%) of internal ringing. The AD204 will then settle to ±0.1% in about 300 ms for a 10 V
AD246
step.
AD202 (IF USED) OR AD204 AD204 –15V C +15V POWER SUPPLY 3.3k V
Figure 13. Output Filter Circuit Showing Proper Grounding
S 0.01 F
Figure 12. Input Filter for Improved Step Response (NOTE: Circuit figures shown on this page are for SIP-style packages. Refer to Page 3 for proper DIP package pinout.) REV. D –7– 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