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OPA2690 Datasheet(PDF) 20 Page - Burr-Brown (TI) |
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OPA2690 Datasheet(HTML) 20 Page - Burr-Brown (TI) |
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20 / 30 page  OPA2690 SBOS238D − JUNE 2002 − REVISED DECEMBER 2004 www.ti.com 20 feedback resistor. This has the interesting advantage that the noise gain becomes equal to 2 for a 50 Ω source impedance—the same as the noninverting circuits considered in the previous section. The amplifier output, however, will now see the 100 Ω feedback resistor in parallel with the external load. In general, the feedback resistor should be limited to the 200 Ω to 1.5kΩ range. In this case, it is preferable to increase both the RF and RG values (see Figure 8), and then achieve the input matching impedance with a third resistor (RM) to ground. The total input impedance becomes the parallel combination of RG and RM. 1/2 O P A 269 0 50 Ω R F 402 Ω R G 200 Ω R B 146 Ω R M 67 Ω Source +5V −5V R O 50 Ω 0.1 µF 6.8 µF + 0.1 µF 0.1 µF 6.8 µF + 50 Ω Load V O V I = −2 V O V I Figure 12. Gain of −2 Example Circuit The second major consideration, touched on in the previous paragraph, is that the signal source impedance becomes part of the noise gain equation and influences the bandwidth. For the example in Figure 12, the RM value combines in parallel with the external 50 Ω source impedance, yielding an effective driving impedance of 50 Ω 67Ω = 28.6Ω. This impedance is added in series with RG for calculating the noise gain (NG). The resultant NG is 2.8 for Figure 12, as opposed to only 2 if RM could be eliminated as discussed above. The bandwidth will therefore be slightly lower for the gain of −2 circuit of Figure 12 than for the gain of +2 circuit of Figure 1. The third important consideration in inverting amplifier design is setting the bias current cancellation resistor on the noninverting input (RB). If this resistor is set equal to the total DC resistance looking out of the inverting node, the output DC error, due to the input bias currents, will be reduced to (Input Offset Current) × RF. If the 50Ω source impedance is DC-coupled in Figure 10, the total resistance to ground on the inverting input will be 228 Ω. Combining this in parallel with the feedback resistor gives the RB = 146Ω used in this example. To reduce the additional high-frequency noise introduced by this resistor, it is sometimes bypassed with a capacitor. As long as RB < 350Ω, the capacitor is not required because the total noise contribution of all other terms will be less than that of the op amp input noise voltage. As a minimum, the OPA2690 requires an RB value of 50Ω to damp out parasitic-induced peaking—a direct short to ground on the noninverting input runs the risk of a very high-frequency instability in the input stage. OUTPUT CURRENT AND VOLTAGE The OPA2690 provides exceptional output voltage and current capabilities in a low-cost monolithic op amp. Under no-load conditions at +25 °C, the output voltage typically swings closer than 1V to either supply rail; the specified swing limit is within 1.2V of either rail. Into a 15 Ω load (the minimum tested load), it will deliver more than ±160mA. The specifications described previously, though familiar in the industry, consider voltage and current limits separately. In many applications, it is the voltage × current, or V-I product, which is more relevant to circuit operation. Refer to the Output Voltage and Current Limitations plot in the Typical Characteristics. The X- and Y-axes of this graph show the zero-voltage output current limit and the zero-current output voltage limit, respectively. The four quadrants give a more detailed view of the OPA2690 output drive capabilities, noting that the graph is bounded by a Safe Operating Area of 1W maximum internal power dissipation for each channel separately. Superimposing resistor load lines onto the plot shows that the OPA2690 can drive ±2.5V into 25Ω or ±3.5V into 50Ω without exceeding the output capabilities or the 1W dissipation limit. A 100 Ω load line (the standard test circuit load) shows the full ±3.9V output swing capability (see the Electrical Characteristics). The minimum specified output voltage and current specifications over temperature are set by worst-case simulations at the cold temperature extreme. Only at cold startup will the output current and voltage decrease to the numbers shown in the Electrical Characteristic tables. As the output transistors deliver power, their junction temperatures increase, decreasing their VBEs (increasing the available output voltage swing) and increasing their current gains (increasing the available output current). In steady-state operation, the available output voltage and current is always greater than that shown in the over-temperature specifications because the output stage junction temperatures will be higher than the minimum specified operating ambient. To protect the output stage from accidental shorts to ground and the power supplies, output short-circuit protection is included in the OPA2690. The circuit acts to limit the maximum source or sink current to approximately 250mA. |
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Similar Description - OPA2690 |
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