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7
ALD1000
TYPICAL PERFORMANCE CURVES
(CONT)
At T
A
= +25
°
C; +V
S
= +24V, –V
S
= –15V, unless otherwise noted.
BASIC OPERATION
ALD1000 FUNCTIONAL BLOCKS
The typical ALD1000 control loop comprises three primary
functional blocks (see Figure 1): the current transmitter
(XTR), the load, and the instrumentation amplifier (IA). The
XTR can be further viewed as divided into the switchable
input operational amplifier (SWOP amp), and the voltage to
current, transconductance amplifier (TA). Each of these
blocks plays a role in the dynamic performance of the
control loop, particularly in terms of loop stability with
reactive loads.
THE CURRENT TRANSMITTER (XTR)
The XTR produces the forward gain necessary for error
amplification. It also controls the frequency response which
must be adjusted to balance the trade-off between step
response and stability when driving reactive loads.
Within the XTR the SWOP amp serves as the input stage. It
amplifies the error between the input and output signals to
produce a precise signal to the TA to drive the load. The
SWOP amp has two pairs of inputs to provide flexibility of
application. The SELECT logic input can switch between
two input and feedback signals. Take care, however, to
insure that the loop remains stable if switching between
current and voltage feedback.
The ALD1000 handles a wide range of load conditions in
either a voltage or current feedback application. The fre-
quency characteristics of the potential load conditions vary
widely. To accommodate these varying frequency character-
istics the XTR includes a compensation network. It consists
of a simple resistor divider network which forms a single
pole, high pass, RC filter when a compensation capacitor is
connected externally.
The transconductance amplifier converts the output voltage
of the SWOP amp into an output current to drive the load.
Whether used in a current feedback or a voltage feedback
loop, the ALD1000 transmitter should be viewed as a source
of current not voltage. In a voltage loop, the output current
is converted to a feedback voltage by the load. In a current
loop the output current is converted to a feedback voltage by
the shunt resistor. The external, XTR gain resistor, tied to E
(Pin 23, Figure 1), sets the voltage to current ratio.
1000
100
10
1
0.1
G
GAIN vs FREQUENCY
1
10
100
1000
10000
Frequency (kHz)
Gain = 5
Gain = 1
Gain = 100
Instrumentation Amplifiers
10
8
6
4
2
0
–2
–4
–6
–8
–10
B
INPUT BIAS CURRENT vs INPUT VOLTAGE
–40
–30
–20
–10
0
10
20
30
40
50
Input Voltage (V)
Gain = 100
Gain = 1
Gain = 100
Gain = 1
C
COMMON-MODE REJECTION vs FREQUENCY
Frequency (Hz)
120
100
80
60
40
20
0
1
10
100
1k
10k
100k
1M
10M
Gain = 100
Gain = 5
Gain = 1
Instrumentation Amplifiers
with ±10V input
25
20
15
10
5
0
–5
–10
–15
C
INPUT COMMON-MODE RANGE vs OUTPUT VOLTAGE
–15
–10
–5
0
5
10
15
20
25
Output Voltage
G = 10
G = 10
G = 1
All Gains
G = 1