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OPA2846 Fiches technique(PDF) 14 Page - Texas Instruments |
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OPA2846 Fiches technique(HTML) 14 Page - Texas Instruments |
14 / 24 page OPA2846 SBOS274A −JUNE 2003 − REVISED MARCH 2004 www.ti.com 14 This will give a −3dB bandwidth approximately equal to: f*3dB + GBP 2 pR FCD Hz The example of Figure 4 will give approximately 44MHz flat bandwidth using the 0.3pF feedback compensation. If the total output noise is bandlimited to a frequency less than the feedback pole frequency, a very simple expression for the equivalent input noise current can be derived as: I EQ + I N 2 ) 4kT R F ) E N R F 2 ) (E N2 pC DF) 2 3 Where: IEQ = Equivalent input noise current if the output noise is bandlimited to F < 1/(2 πRFCF) IN = Input current noise for the op amp inverting input EN = Input voltage noise for the op amp CD = Diode capacitance F = Bandlimiting frequency in Hz (usually a post filter prior to further signal processing) Evaluating this expression up to the feedback pole frequency at 31MHz for the circuit of Figure 4 gives an equivalent input noise current of 3.1pA/Hz. This is only slightly higher than the current noise of the op amp itself. TWO-STAGE TRANSIMPEDANCE DESIGN The dual OPA2846 may be used as either a dual transimpedance channel from two photodetectors, or as a very high gain stage by using one amplifier as the transimpedance stage with the second used as a post gain amplifier. See Figure 5 for an example of using one channel as a transimpedance front end from a large area detector, with the second amplifier used as a voltage gain stage to get a 100k Ω total gain (ZT) from a large 50pF detector (CD in Figure 5). 2.67k Ω 20 Ω 20 Ω 732 Ω 2.67k Ω 1/2 OP A 2 84 6 1000pF C D 50pF 1.9pF 1/2 OP A 28 46 −V B λ Figure 5. High-Gain, Wideband Transimpedance Amplifier One key question in this design is how best to split up the first and second stage gains. If bandwidth optimization from a given photodetector capacitance (CD in Figure 5) is the primary goal, Equation 4 gives a solution for RF in the input stage that will provide an equal bandwidth in the first and second stages, giving the maximum overall channel bandwidth. R F + Z T 2 2 pC D GBP Where: ZT = Desired total transimpedance gain CD = Diode capacitance at reverse bias GBP = Amplifier Gain Bandwidth Product (MHz) This equation is used to calculate the required input stage feedback resistor in Figure 5. The remaining total signal gain is provided by the second stage; in the example of Figure 5, setting G = 37.5 gives the same bandwidth (approximately 44MHz) as the bandwidth achieved by the input stage. To set this first stage bandwidth to its maximally flat values, use Equation 5 to set the feedback capacitor value: C F + C D pR F GBP f*3dB + 1 2 (GBP) 2 3 (2 pC D) 1 3 (Z T) 1 3 The approximate achievable bandwidth in the two stages is given by Equation 6, which gives approximately 30MHz for Figure 5. LOW-GAIN COMPENSATION FOR IMPROVED SFDR Where a low gain is desired, and inverting operation is acceptable, a new external compensation technique may be used to retain the full slew rate and noise benefits of the OPA2846 while giving increased loop gain and the associated improvement in distortion offered by the decompensated architecture. This technique shapes the loop gain for good stability while giving an easily-con- trolled, 2nd-order, low-pass frequency response. Consid- ering only the noise gain (noninverting signal gain, which is also called the Noise Gain or NG) for the circuit of Figure 6, the low-frequency noise gain, (NG1) will be set by the resistor ratios while the high-frequency noise gain (NG2) will be set by the capacitor ratios. The capacitor values set both the transition frequencies and the high-frequency noise gain. If this noise gain (determined by NG2 = 1 + CS/CF) is set to a value greater than the recommended minimum stable gain for the op amp, and the noise gain pole (set by 1/RFCF) is placed correctly, a very well-controlled, 2nd-order, low-pass frequency response will result. (2) (3) (4) (5) (6) |
Numéro de pièce similaire - OPA2846 |
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Description similaire - OPA2846 |
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