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參數(shù)資料
| 型號： | ALD1000U |
| 英文描述： | IC-SM-PROG LINE DRIVER |
| 中文描述： | 集成電路釤PROG線路驅(qū)動器 |
| 文件頁數(shù)： | 10/14頁 |
| 文件大小： | 209K |
| 代理商： | ALD1000U |
10
ALD1000
insure adequate transient response and loop stability: loop
gain and phase. Together loop gain and phase set the phase
margin which defines dynamic performance.
Loop gain is the product of the forward voltage to current
ratio, the load impedance, and the IA gain. The input error
voltage is converted to an output current. The output current
is converted to a feedback voltage by the load impedance.
The feedback voltage is gained up by the feedback IA. All
three blocks affect loop stability.
The XTR gain resistor, which is connected to the E pin of the
ALD1000, adjusts the voltage to current relationship. In-
creasing this resistor decreases loop gain. This, in turn,
increases phase margin and slows step response. This resis-
tor will typically be between 250
and 2500
.
In a voltage feedback loop the frequency at which the loop
gain starts to roll off decreases with increasing capacitance.
It is necessary to compensate for the loss of bandwidth
caused by load capacitance. The compensation network
provides this capability. Typical performance curve “Com-
pensation Capacitor vs Load Capacitance” illustrates typical
compensation capacitor values for load capacitance varying
from 1pf to 1
μ
f. Exact capacitor values will vary with the
load resistance, the XTR gain resistor value, IA gain, and
variability of the open loop gain of the ALD1000 SWOP
amp. This curve provides a starting point for empirical
selection of the compensation capacitor value.
The effect described above is much less significant with a
current feedback loop since the shunt resistor’s capacitance
can be easily controlled. The current feedback loop will be
more robust when load conditions are unknown or varying.
LOOP STABILITY AND THE INSTRUMENTATION
AMPLIFIERS
The frequency characteristics and gain of the instrumenta-
tion amplifiers affect loop stability when they are used in a
feedback loop. There are two main contributions. First, the
IA gain directly multiplies loop gain. As a result high IA
gains reduce phase margin. Second, when the input exceeds
the IA range the IA output can no longer provide the
necessary feedback. This can result in a lock condition.
Both of these situations are discussed further below.
LOOP GAIN AND THE INSTRUMENTATION
AMPLIFIERS
The ALD1000 is designed for use in a feedback loop. When
one of the instrumentation amplifiers is used as the feedback
amplifier its gain directly contributes to loop gain. The loop
can become unstable if the loop gain is too large. Con-
versely, it may be possible to stabilize a difficult loop by
reducing the gain of the IA.
Refer to Figure 4. In this circuit the ALD1000 is configured
in a current loop with a 50
shunt resistor. A 20ma full scale
current through the 50
shunt results in a 1V feedback
signal. The IA must remove the common-mode level from
the shunt voltage and scale the resulting differential signal
up to the input signal level.
limit this range when a differential input voltage causes the
output voltage to increase. Thus, the linear common-mode
range relates to the output voltage of the complete amplifier.
This behavior also depends in supply voltage—see perfor-
mance curve “Input Common-Mode Range vs Output Volt-
age.”
The combination of a significant differential signal and a
high common-mode voltage as occurs in the current feed-
back configuration reduces the common-mode range. Ex-
ceeding the common-mode range results in a reduced IA
output voltage. When this occurs the feedback loop can no
longer balance. The forward gain of the ALD1000 amplifies
this false error signal, the output voltage tries to increase,
and this holds the IA in an overloaded condition.
The ALD1000 applies two defenses against this problem.
First, there is a 100
resistor in series with the transmitter
output. This resistor, which primarily provides protection
from over-voltage damage to the output terminal, acts to
limit the output swing under high current conditions. Sec-
ond, the ALD1000’s error detection circuitry signals when
the transmitter output voltage exceeds rating. This serves to
detect a potential lock condition.
Limiting the transmitter’s output swing to within the instru-
mentation amplifier’s input range allows the loop to recover
without reducing the input signal should a transient voltage
level exceed the common-mode input range. However, the
common-mode range of the instrumentation amplifiers var-
ies with application specific factors. Lock-up can occur. The
application designer must provide defenses against this
condition where it is warranted.
USING THE INSTRUMENTATION AMPLIFIERS WITH
A FLOATING SIGNAL SOURCE
The input impedance of the ALD1000 instrumentation am-
plifiers are very high—about 10
6
. Within a feedback
loop, as shown in the examples, this characteristic acts to
minimize errors caused by loading of the feedback signal.
However, if used as an amplifier for a thermocouple, micro-
phone, or other isolated signal source a path is needed for the
input bias current. This current is nominally about 100nA.
Without a return path the inputs will float to a potential that
exceeds the common-mode range of the amplifier. See
Figure 10.
LOOP STABILITY
The stability of a closed loop system such as the intended
application of the ALD1000 requires adequate phase mar-
gin. In contrast, excessive phase margin will reduce the
circuit’s transient response to fast changing signals. It is the
intent of this section to give an insight into how the ALD1000
circuits blocks affect dynamic performance. Selection of the
loop architecture and compensation can then be done em-
pirically.
LOOP STABILITY AND THE XTR
There are two critical parameters that must be controlled to
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