OPA4684M Datasheet, PDF (14/32 Pages) Texas Instruments – QUAD LOW-POWER CURRENT-FEEDBACK OPERATIONAL AMPLIFIER

OPA4684M
QUAD LOWĆPOWER CURRENTĆFEEDBACK
OPERATIONAL AMPLIFIER
SGLS145B − AUGUST 2003 − REVISED FEBRUARY 2004
Figure 42. DC-Coupled, G = _1V/V, Bipolar Supply Specifications and Test Circuit
These circuits show ±5V operation. The same circuits can be applied with bipolar supplies from ±2.5V to ±6V.
Internal supply independent biasing gives nearly the same performance for the OPA4684 over this wide range
of supplies. Generally, the optimum feedback resistor value (for nominally flat frequency response at G = +2)
will increase in value as the total supply voltage across the OPA4684 is reduced.
See Figure 43 for the AC-coupled, single +5V supply, gain of +2V/V circuit configuration used as a basis for the
+5V only Electrical and Typical Characteristics for each channel. The key requirement of broadband
single-supply operation is to maintain input and output signal swings within the usable voltage ranges at both
the input and the output. The circuit of Figure 43 establishes an input midpoint bias using a simple resistive
divider from the +5V supply (two 10-kΩ resistors) to the noninverting input. The input signal is then AC-coupled
into this midpoint voltage bias. The input voltage can swing to within 1.25 V of either supply pin, giving a 2.5-Vp-p
input signal range centered between the supply pins. The input impedance of Figure 43 is set to give a 50ĆΩ
input match. If the source does not require a 50ĆΩ match, remove this and drive directly into the blocking
capacitor. The source will then see the 5-kΩ load of the biasing network as a load. The gain resistor (R_G) is
AC-coupled, giving the circuit a DC gain of +1, which puts the noninverting input DC bias voltage (2.5 V) on the
output as well. The feedback resistor value has been adjusted from the bipolar ±5V supply condition to
re-optimize for a flat frequency response in +5V only, gain of +2, operation. On a single +5V supply, the output
voltage can swing to within 1.0 V of either supply pin while delivering more than 70-mA output current—easily
giving a 3-Vp-p output swing into 100 Ω (8dBm maximum at the matched 50ĆΩ load). The circuit of Figure 43
shows a blocking capacitor driving into a 50ĆΩ output resistor, then into a 50ĆΩ load. Alternatively, the blocking
capacitor could be removed if the load is tied to a supply midpoint or to ground if the DC current required by the
load is acceptable.
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