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參數(shù)資料
| 型號(hào)： | AB-101 |
| 英文描述： | AB-101 - COMBINING AN AMPLIFIER WITH THE BUF634 |
| 中文描述： | 抗體- 101 -相結(jié)合BUF634放大器 |
| 文件頁數(shù)： | 3/6頁 |
| 文件大小： | 69K |
| 代理商： | AB-101 |
3
by the on-resistance of the multiplexer, which can vary
between several ohms and several kilo ohms, depending
upon the type. If a filter capacitor is located in front of the
multiplexer, it functions as a low-impedance voltage source.
Figure 5 shows the measurements from this configuration.
It is interesting to note that this current can only flow into the
input stage because of the protection circuitry of the bipolar
transistors. In FET op amps, this protection is not necessary
and consequently no current flows out of the signal source.
Comparing the input circuits of bipolar and FET op amps
clearly illustrates this distinction. While the transistor’s
reversed base-emitter junction breaks at about 7V, the blocked
gate channel diode of a J-FET can handle more than 30V.
CONSEQUENCES FOR THE INPUT FILTER
If a bipolar op amp is inserted after the multiplexer with
preceding low-pass filter, the overload input current flows
from the filter capacitor into the amplifier circuit, as
shown in Figure 5. Here, a simple example shows the size
of this error:
In the first approximation, the error current (I
IN
) curve re-
sembles a sawtooth shape. The peak value, as shown in Figure
5, is:
10V – 0.7V
10k
=
1.5k
≈
0.8mA
The settling time should be about 3
μ
s (linear ramp):
Therefore, the filter capacitor will change its voltage by
12mV with 3
μ
s. This does not include the recharging current
from the voltage via R
1
. Note:
τ
is the time constant for the
time required to recharge the last charge. Unfortunately,
however, this recharging current is almost neglectable be-
cause of the long time constant:
Thus, compensating for the initial droop would take a
relatively long time.
As a result, the voltage difference of the channels switched
one after another by the MUX produces a corresponding
overload error current in the amplifier, which results in a lot
of crosstalk.
GAIN PERFORMANCE
The large current already mentioned also flows into or
through the protection circuitry, especially at large signal
levels. For this reason, the important factor is the voltage
threshold of the protection diodes. At an input voltage of
only 1V and a corresponding gain of 10V/V, the protection
diodes are conductive only for a small voltage range. On the
FIGURE 6. Current Path Through the Protection Diodes.
other hand, in this multiplexer configuration, a current flows
through R
1
and R
F
, as shown in Figure 6. The voltage error
from this current, which arises at the source, is also gained
by a factor of 10. This error current disappears when input
voltage steps are less than 0.7V, which is lower than the
response threshold of the protection diodes.
ELIMINATING ERROR
The voltage change at the filter capacitor is a product of the
error current, the settling time, and the size of C
1
. If R
F
is
selected ten times larger, the error decreases by about 1/10
to 1.2mV. For high-resolution converters, this error is still
too large, and an R
F
of 100k
is also a significant error
source for bipolar op amps. On the other hand, it is also
possible to enlarge C
1
, although the mechanical dimensions
of 1
μ
F to 4.7
μ
F set a limit to the value of capacitor
selected. A compromise of C
1
= 2
μ
F and R
F
= 25k
produces an acceptable error of about 0.3mV, based on a
10V voltage difference between the channels and a circuit
as shown in Figure 5.
If, however, R
1
and C
1
are significantly decreased to enable
recharging from the source, the effect of the low-pass filter
and its protection decrease greatly. Even more important, the
current pulse in this case would flow across the supply line
between the source and the filter input, which is undesirable
for other reasons.
The error current can also be reduced using a current-
limiting circuit. This type of circuit is described under the
section “Tips”. For better error elimination, a FET op amp
can be used. As already mentioned, this op amp does not
allow the error current to increase at all.
INSTRUMENTATION AMPLIFIER
If the threshold voltage of the protection diodes is known, it
is easy to calculate the overload input current of an op amp
circuit. With instrumentation amplifiers (INAs), the input
protection circuitry is not always recognizable.
τ =
10k
100nF
=
1ms!
V
C
=
I
t
C
2
=
0.8mA
3
μ
s
100nF
2
=
12mV
I
E
=
R
F
R
1
V
OUT
V
IN
I
IN
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