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OPA2680N Datasheet(PDF) 11 Page - Burr-Brown (TI) |
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OPA2680N Datasheet(HTML) 11 Page - Burr-Brown (TI) |
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11 / 21 page  ® OPA2680 11 APPLICATIONS INFORMATION WIDEBAND VOLTAGE FEEDBACK OPERATION The OPA2680 provides an exceptional combination of high output power capability in a dual, wideband, unity gain stable voltage feedback op amp using a new high slew rate input stage. Typical differential input stages used for voltage feedback op amps are designed to steer a fixed-bias current to the compensation capacitor, setting a limit to the achiev- able slew rate. The OPA2680 uses a new input stage which places the transconductance element between two input buffers, using their output currents as the forward signal. As the error voltage increases across the two inputs, an increas- ing current is delivered to the compensation capacitor. This provides very high slew rate (1800V/ µs) while consuming relatively low quiescent current (6.4mA/ch.). This excep- tional full power performance comes at the price of a slightly higher input noise voltage than alternative architectures. The 4.8nV/ √Hz input voltage noise for the OPA2680 is excep- tionally low for this type of input stage. Figure 1 shows the DC-coupled, gain of +2, dual power supply circuit configuration used as the basis of the ±5V Specifications and Typical Performance Curves. This is for one channel. The other channel is connected similarly. For test purposes, the input impedance is set to 50 Ω with a resistor to ground and the output impedance is set to 50 Ω with a series output resistor. Voltage swings reported in the specifications are taken directly at the input and output pins, while output powers (dBm) are at the matched 50 Ω load. For the circuit of Figure 1, the total effective load will be 100 Ω || 804 Ω. The disable control line (SO-14 package only) is typically left open to guarantee normal amplifier operation. Two optional components are included in Figure 1. An additional resistor (175 Ω) is included in series with the non- inverting input. Combined with the 25 Ω DC source resis- tance looking back towards the signal generator, this gives an input bias current cancelling resistance that matches the 200 Ω source resistance seen at the inverting input (see the DC Accuracy and Offset Control section). In addition to the usual power supply decoupling capacitors to ground, a 0.1 µF capacitor is included between the two power supply pins. In practical PC board layouts, this optional-added capacitor will typically improve the 2nd harmonic distortion performance by 3dB to 6dB. Figure 2 shows the AC-coupled, gain of +2, single supply circuit configuration which is the basis of the +5V Specifi- cations and Typical Performance Curves. Though not a “rail- to-rail” design, the OPA2680 requires minimal input and output voltage headroom compared to other very wideband voltage feedback op amps. It will deliver a 3Vp-p output swing on a single +5V supply with >150MHz bandwidth. The key requirement of broadband single-supply operation is to maintain input and output signal swings within the useable voltage ranges at both the input and the output. The circuit of Figure 2 establishes an input midpoint bias using a simple resistive divider from the +5V supply (two 698 Ω resistors). Separate networks would be required at each input. The input signal is then AC-coupled into the midpoint voltage bias. The input voltage can swing to within 1.5V of either supply pin, giving a 2Vp-p input signal range centered between the supply pins. The input impedance matching resistor (59 Ω) used for testing is adjusted to give a 50Ω input load when the parallel combination of the biasing divider network is included. Again, an additional resistor (50 Ω in this case) is included directly in series with the non-inverting input. This minimum recommended value provides part of the DC source resistance matching for the non-inverting input bias current. It is also used to form a simple parasitic pole to roll off the frequency response at very high frequen- cies (>500MHz) using the input parasitic capacitance. The gain resistor (RG) is AC-coupled, giving the circuit a DC gain of +1, which puts the input DC bias voltage (2.5V) on the output as well. The output voltage can swing to within 1V of either supply pin while delivering >100mA output FIGURE 1. DC-Coupled, G = +2, Bipolar Supply, Specifi- cation and Test Circuit. FIGURE 2. AC-Coupled, G = +2, Single Supply Specifica- tion and Test Circuit. 1/2 OPA2680 +5V + DIS –5V 50 Ω Load 50 Ω 175 Ω 50 Ω V O V D V I 50 Ω Source R G 402 Ω R F 402 Ω + 6.8µF 0.1µF 6.8µF 0.1µF 0.1µF 1/2 OPA2680 +5V +V S DIS V S/2 698 Ω 100 Ω V O V D V I 698 Ω 50 Ω R G 402 Ω R F 402 Ω 0.1µF 0.1µF 59 Ω + 6.8µF 0.1µF |
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