OPA4684M Datasheet, PDF (24/32 Pages) Texas Instruments – QUAD LOW-POWER CURRENT-FEEDBACK OPERATIONAL AMPLIFIER

OPA4684M
QUAD LOWĆPOWER CURRENTĆFEEDBACK
OPERATIONAL AMPLIFIER
SGLS145B − AUGUST 2003 − REVISED FEBRUARY 2004
R_I, the buffer output impedance, is a critical portion of the bandwidth control equation. The OPA4684 reduces
this element to approximately 4.0 Ω using the local loop gain of the input buffer stage. This significant reduction
in output impedance, on very low power, contributes significantly to extending the bandwidth at higher gains.
A current-feedback op amp senses an error current in the inverting node (as opposed to a differential input error
voltage for a voltage-feedback op amp) and passes this on to the output through an internal
frequency-dependent transimpedance gain. The Typical Characteristics show this open-loop transimpedance
response. This is analogous to the open-loop voltage gain curve for a voltage-feedback op amp. Developing
the transfer function for the circuit of Figure 53 gives Equation 1:
VO
VI
+
ǒ Ǔ a
1
)
R_F
R_G
ǒ Ǔ R_F)R_I 1)_RRGF
+ ^aNG
1
)
R_F)R_I
Z_(s)
NG
ƪ ǒ Ǔƫ NG +
1
)
RF
RG
(1)
1)
Z_(s)
This is written in a loop-gain analysis format where the errors arising from a non-infinite open-loop gain are
shown in the denominator. If Z_(S) were infinite over all frequencies, the denominator of Equation 1 would reduce
to 1 and the ideal desired signal gain shown in the numerator would be achieved. The fraction in the denominator
of Equation 1 determines the frequency response. Equation 2 shows this as the loop-gain equation.
RF
Z_(s)
) RI
NG
+
Loop
Gain
(2)
If 20 × log(RF + NG × RI) were drawn on top of the open-loop transimpedance plot, the difference between the
two would be the loop gain at a given frequency. Eventually, Z(S) rolls off to equal the denominator of Equation
2 at which point the loop gain has reduced to 1 (and the curves have intersected). This point of equality is where
the amplifier’s closed-loop frequency response given by Equation 1 will start to roll off, and is exactly analogous
to the frequency at which the noise gain equals the open-loop voltage gain for a voltage feedback op amp. The
difference here is that the total impedance in the denominator of Equation 2 may be controlled somewhat
separately from the desired signal gain (or NG).
The OPA4684 is internally compensated to give a maximally flat frequency response for RF = 800 Ω at
NG = 2 on ±5-V supplies. That optimum value goes to 1.0 kΩ on a single +5V supply. Normally, with a
current-feedback amplifier, it is possible to adjust the feedback resistor to hold this bandwidth up as the gain
is increased. The CFBPLUS architecture has reduced the contribution of the inverting input impedance to
provide exceptional bandwidth to higher gains without adjusting the feedback resistor value. The Typical
Characteristics show the small-signal bandwidth over gain with a fixed feedback resistor.
Putting a closed-loop buffer between the noninverting and inverting inputs does bring some added
considerations. Since the voltage at the inverting output node is now the output of a locally closed-loop buffer,
parasitic external capacitance on this node can cause frequency response peaking for the transfer function from
the noninverting input voltage to the inverting node voltage. While it is always important to keep the inverting
node capacitance low for any current-feedback op amp, it is critically important for the OPA4684. External layout
capacitance in excess of 2 pF will start to peak the frequency response. This peaking can be easily reduced
by then increasing the feedback resistor value−but it is preferable, from a noise and dynamic range standpoint,
to keep that capacitance low, allowing a close to nominal 800-Ω feedback resistor for flat frequency response.
Very high parasitic capacitance values on the inverting node (> 5 pF) can possibly cause input stage oscillation
that cannot be filtered by a feedback element adjustment.
At very high gains, 2nd-order effects in the inverting output impedance cause the overall response to peak up.
If desired, it is possible to retain a flat frequency response at higher gains by adjusting the feedback resistor
to higher values as the gain is increased. Since the exact value of feedback that will give a flat frequency
response depends strongly in inverting and output node parasitic capacitance values, it is best to experiment
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