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OPA656N250G4 Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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OPA656N250G4 Datasheet(HTML) 11 Page - National Semiconductor (TI) |
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11 / 25 page  OPA656 11 SBOS196G www.ti.com is peaked up over frequency by the diode source capaci- tance, and can, in many cases, become the limiting factor to input sensitivity. The key elements to the design are the expected diode capacitance (CD) with the reverse bias volt- age (–VB) applied, the desired transimpedance gain, RF, and the GBP for the OPA656 (230MHz). Figure 3 shows a design from a 25pF source capacitance diode through a 50k Ω transimpedance gain. With these 3 variables set (including the parasitic input capacitance for the OPA656 added to CD), the feedback capacitor value (CF) may be set to control the frequency response. If the total output noise is bandlimited to a frequency less than the feedback pole frequency (1/RFCF), a very simple expression for the equivalent input noise current can be derived as: II kT R E R EC F EQ N F N F ND =+ + + ( ) 2 2 2 4 2 3 π Where: iEQ = Equivalent input noise current if the output noise is bandlimited to F < 1/(2 πR FCD). 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 postfilter prior to further signal processing). 4kT = 1.6E – 20J at 290 °K. Evaluating this expression up to the feedback pole frequency at 3.8MHz for the circuit of Figure 3, gives an equivalent input noise current of 2.7pA/ Hz . This is much higher than the 1.3fA/ Hz for just the op amp itself. This result is being dominated by the last term in the equivalent input noise current expression. It is essential in this case to use a low voltage noise op amp. DESIGN-IN TOOLS DEMONSTRATION FIXTURES Two printed circuit boards (PCBs) are available to assist in the initial evaluation of circuit performance using the OPA656 in its two package options. Both of these are offered free of charge as unpopulated PCBs, delivered with a user's guide. The summary information for these fixtures is shown in Table I. To achieve a maximally flat 2nd-order Butterworth frequency response, the feedback pole should be set to: 12 4 /( ) ( /( )) ππ RC GBP RC FF FD = Adding the common mode and differential mode input ca- pacitance (0.7 + 2.8)pF to the 25pF diode source capaci- tance of Figure 3, and targeting a 50k Ω transimpedance gain using the 230MHz GBP for the OPA656 will require a feedback pole set to 3.8MHz. This will require a total feed- back capacitance of 0.8pF. Typical surface-mount resistors have a parasitic capacitance of 0.2pF leaving the required 0.6pF value shown in Figure 3 to get the required feedback pole. This will give an approximate –3dB bandwidth set by: f GBP R C Hz dB F D − = 3 2 /) π The example of Figure 3 will give approximately 5.7MHz flat bandwidth using the 0.6pF feedback compensation. FIGURE 3. Wideband, Low-Noise, Transimpedance Amplifier. R F 50k Ω Supply Decoupling Not Shown C D 25pF λ OPA656 +5V –5V –V B I D V O = ID RF C F 0.6pF ORDERING LITERATURE PRODUCT PACKAGE NUMBER NUMBER OPA656U SO-8 DEM-OPA-SO-1A SBOU009 OPA656N SOT23-5 DEM-OPA-SOT-1A SBOU010 TABLE I. Demonstration Fixtures by Package. The demonstration fixtures can be requested at the Texas Instruments web site (www.ti.com) through the OPA656 product folder. |
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