Datasheet LT1008 (Analog Devices) - 9
| Manufacturer | Analog Devices |
| Description | Picoamp Input Current, Microvolt Offset, Low Noise Op Amp |
| Pages / Page | 16 / 9 — APPLICATIO S I FOR ATIO. 0.1Hz to 10Hz Noise Test Circuit. Frequency … |
| File Format / Size | PDF / 283 Kb |
| Document Language | English |
APPLICATIO S I FOR ATIO. 0.1Hz to 10Hz Noise Test Circuit. Frequency Compensation. Inverter Feedforward Compensation
Model Line for this Datasheet
Text Version of Document
LT1008
U U W U APPLICATIO S I FOR ATIO 0.1Hz to 10Hz Noise Test Circuit
0.1μF 100k 10Ω – 2k LT1008* + 22μF + 4.3k SCOPE 4.7μF LT1001 ×1 VOLTAGE R – 2.2μF 110k IN = 1M GAIN: 50,000 100k *DEVICE UNDER TEST 24.3k NOTE: ALL CAPACITOR VALUES ARE FOR NONPOLARIZED CAPACITORS ONLY 0.1μF 1008 AI03
Frequency Compensation
In the voltage follower configuration, when the input is driven by a fast, large-signal pulse (>1V), the input protec- The LT1008 is externally frequency compensated with a tion diodes effectively short the output to the input during single capacitor. The two standard compensation circuits slewing, and a current, limited only by the output short- shown earlier are identical to the LM108A/LM308A fre- circuit protection, will flow through the diodes. quency compensation schemes. Therefore, the LT1008 operational amplifiers can be inserted directly into The use of a feedback resistor, as shown in the voltage LM108A/LM308A sockets, with similar AC and upgraded follower feedforward diagram, is recommended because DC performance. this resistor keeps the current below the short-circuit limit, resulting in faster recovery and settling of the output. External frequency compensation provides the user with additional flexibility in shaping the frequency response of
Inverter Feedforward Compensation
the amplifier. For example, for a voltage gain of ten and C2 C 5pF 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 R1 R2 rate increases to 4V/μs. The LT1008 can also be overcom- 10k 2 INPUT – 10k pensated (i.e., C 6 F > 30pF or CS > 100pF) to improve capaci- LT1008 VOUT tive load handling capability or to narrow noise bandwidth. 3 + 8 In many applications, the feedback loop around the ampli- 1 R3 3k fier has gain (e.g., logarithmic amplifiers); overcompen- C3 sation can stabilize these circuits with a single capacitor. 10pF C1 500pF 1008 AI05 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. 2V/DIV 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. 5μs/DIV 1008 AI07 1008fb 9
U U W U APPLICATIO S I FOR ATIO 0.1Hz to 10Hz Noise Test Circuit
0.1μF 100k 10Ω – 2k LT1008* + 22μF + 4.3k SCOPE 4.7μF LT1001 ×1 VOLTAGE R – 2.2μF 110k IN = 1M GAIN: 50,000 100k *DEVICE UNDER TEST 24.3k NOTE: ALL CAPACITOR VALUES ARE FOR NONPOLARIZED CAPACITORS ONLY 0.1μF 1008 AI03
Frequency Compensation
In the voltage follower configuration, when the input is driven by a fast, large-signal pulse (>1V), the input protec- The LT1008 is externally frequency compensated with a tion diodes effectively short the output to the input during single capacitor. The two standard compensation circuits slewing, and a current, limited only by the output short- shown earlier are identical to the LM108A/LM308A fre- circuit protection, will flow through the diodes. quency compensation schemes. Therefore, the LT1008 operational amplifiers can be inserted directly into The use of a feedback resistor, as shown in the voltage LM108A/LM308A sockets, with similar AC and upgraded follower feedforward diagram, is recommended because DC performance. this resistor keeps the current below the short-circuit limit, resulting in faster recovery and settling of the output. External frequency compensation provides the user with additional flexibility in shaping the frequency response of
Inverter Feedforward Compensation
the amplifier. For example, for a voltage gain of ten and C2 C 5pF 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 R1 R2 rate increases to 4V/μs. The LT1008 can also be overcom- 10k 2 INPUT – 10k pensated (i.e., C 6 F > 30pF or CS > 100pF) to improve capaci- LT1008 VOUT tive load handling capability or to narrow noise bandwidth. 3 + 8 In many applications, the feedback loop around the ampli- 1 R3 3k fier has gain (e.g., logarithmic amplifiers); overcompen- C3 sation can stabilize these circuits with a single capacitor. 10pF C1 500pF 1008 AI05 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. 2V/DIV 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. 5μs/DIV 1008 AI07 1008fb 9
