LT1008_1 Datasheet, PDF (9/16 Pages) Linear Technology – Picoamp Input Current, Microvolt Offset, Low Noise Op Amp

LT1008
APPLICATIO S I FOR ATIO
0.1Hz to 10Hz Noise Test Circuit
0.1μF
100k
10Ω
–
LT1008*
+
2k
4.7μF
VOLTAGE
GAIN: 50,000
*DEVICE UNDER TEST
NOTE: ALL CAPACITOR VALUES ARE FOR
NONPOLARIZED CAPACITORS ONLY
+
4.3k
LT1001
–
2.2μF
100k
24.3k
0.1μF
22μF
SCOPE
×1
110k ^{RIN = 1M}
1008 AI03
Frequency Compensation
The LT1008 is externally frequency compensated with a
single capacitor. The two standard compensation circuits
shown earlier are identical to the LM108A/LM308A fre-
quency compensation schemes. Therefore, the LT1008
operational amplifiers can be inserted directly into
LM108A/LM308A sockets, with similar AC and upgraded
DC performance.
External frequency compensation provides the user with
additional flexibility in shaping the frequency response of
the amplifier. For example, for a voltage gain of ten and
C_F = 3pF, a gain bandwidth product of 5MHz and slew rate
of 1.2V/μs can be realized. For closed-loop gains in excess
of 200, no external compensation is necessary, and slew
rate increases to 4V/μs. The LT1008 can also be overcom-
pensated (i.e., C_F > 30pF or C_S > 100pF) to improve capaci-
tive load handling capability or to narrow noise bandwidth.
In many applications, the feedback loop around the ampli-
fier has gain (e.g., logarithmic amplifiers); overcompen-
sation can stabilize these circuits with a single capacitor.
The availability of the compensation terminals permits the
use of feedforward frequency compensation to enhance
slew rate in low closed-loop gain configurations. The
inverter slew rate is increased to 1.4V/μs. The voltage
follower feedforward scheme bypasses the amplifier’s
gain stages and slews at nearly 10V/μs.
The inputs of the LT1008 are protected with back-to-back
diodes. Current limiting resistors are not used, because the
leakage of these resistors would prevent the realization of
picoampere level bias currents at elevated temperatures.
In the voltage follower configuration, when the input is
driven by a fast, large-signal pulse (>1V), the input protec-
tion diodes effectively short the output to the input during
slewing, and a current, limited only by the output short-
circuit protection, will flow through the diodes.
The use of a feedback resistor, as shown in the voltage
follower feedforward diagram, is recommended because
this resistor keeps the current below the short-circuit
limit, resulting in faster recovery and settling of the output.
Inverter Feedforward Compensation
C2
5pF
R1
10k
INPUT
R2
2_–
10k
6
LT1008
3₊
8
1
R3
3k
C1
500pF
C3
10pF
1008 AI05
V_OUT
5μs/DIV
1008 AI07
1008fb
9