How to achieve weak signal amplification?
Sensor + OP amp +adc+ processor is a typical op amp application circuit, in this application, a typical problem is that the sensor provides a very low current, in this case, how to complete the signal amplification?
Most fixed frequency signals (modulated signals) can be detected by the coherence of the signal. But most of the signal of the scene sensor is not modulated signal, and the algorithm can not be detected by coherence.
How to improve signal-to-noise ratio of signals by circuit design
Another engineer friend suggested that special attention should be paid to selecting high impedance, low noise operation and low noise resistance in the selection of op amps, capacitors, resistors, and cloth plates. Design considerations are as follows:
Point one, prevent interference:
1) The circuit design to pay attention to the balance of treatment, as far as possible, for the suppression of interference effective, these in the U.S. National Semiconductor, BB (has been acquired by Ti), Adi and other companies on the OP amp design manuals can be found.
2) recommended to add metal shielding cover, the weak signal part of the cover up (open a small mold), metal body connected to the circuit, can greatly improve the circuit anti-jamming ability. If it is a large impedance sensor, the input impedance of the amplification circuit is also large impedance, the circuit itself is extremely susceptible to interference,
Second, improve the device background noise:
1) for the NA level of the sensor output, select the op amp of the input current PA level. If the speed does not require much, the OP amp is not expensive. Instrumentation amplifiers Of course, the best, is the higher cost.
2) If the use of non-instrument op amp, the feedback resistor will not be too big, M-euro better. Otherwise, the resistance requirements are relatively high. The rear stage is then 2-stage amplification, and a simple high-pass circuit is added in the middle to suppress 50Hz interference.
3) Pay attention to the noise density of your signal using the band, not just to see the full band noise
For example, the signal noise of the LT1028 is relatively low at low frequencies, but
Voltagenoise
1.1nv/√hz Max at 1kHz
0.85nv/√hz Typ at 1kHz
1.0nv/√hz Typ at 10Hz
35nvp-ptyp, 0.1Hz to 10Hz
Only the noise of frequencies below 1k is noted, and no higher frequency noise is noted.
Main points three, circuit design:
Choose the op amp that satisfies the signal frequency as much as possible, and do not introduce unnecessary noise from the band; depending on your frequency band, magnification, select the most appropriate op amp for the gain bandwidth product.
Reduce the cutoff frequency of the circuit (taking full account of bandwidth requirements, phase requirements) using a transimpedance shunt capacitor
The noise can be reduced by the following circuits, but only if the source noise is greater than the device floor noise, otherwise it is counterproductive.
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Hardware 100,000 why--op-amp chapter (iv) weak signal amplification techniques