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

Ω and 2500Ω.

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

µf. 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

Ω shunt resistor. A 20ma full scale

current through the 50

Ω 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

Ω 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-

Ω. 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