OPA2686 Datasheet, PDF (15/18 Pages) Burr-Brown (TI) – Dual, Wideband, Low Noise, Voltage Feedback OPERATIONAL AMPLIFIER

OPERATING SUGGESTIONS
SETTING RESISTOR VALUES TO MINIMIZE NOISE
The OPA2686 provides a very low input noise voltage while
requiring a low 12mA/channel quiescent current. To take
full advantage of this low input noise, careful attention to
the other possible noise contributors is required. Figure 11
shows the op amp noise analysis model with all the noise
terms included. In this model, all the noise terms are taken
to be noise voltage or current density terms in either nV/√Hz
or pA/√Hz.
E_NI
1/2
R_S
I_BN
OPA2686
E_O
E_RS
√4kTR_S
4kT
R_G
R_F
√4kTR_F
R_G
I_BI
4kT = 1.6E –20J
at 290°K
FIGURE 11. Op Amp Noise Analysis Model.
The total output spot noise voltage can be computed as the
square root of the squared contributing terms to the output
noise voltage. This computation adds all the contributing
noise powers at the output by superposition, then takes the
square root to get back to a spot noise voltage. Equation 11
shows the general form for this output noise voltage using
the terms shown in Figure 11.
Eq. 11
( ) ^{( ) ( ) E}O ⁼
E
2
NI
+
I _BN R _S
2 _{+ 4kTRS}
NG² +
I_BIR _F
2 _{+ 4kTRFNG}
Dividing this expression by the noise gain (NG = 1+R_F/R_G)
will give the equivalent input-referred spot noise voltage at
the non-inverting input as shown in Equation 12.
Eq. 12
( ) E_N =
E_NI2 +
I _BN R _S
2
+
4kTR_S
+


I _BI R _F
NG
²

+
4 kTR _F
NG
Inserting high resistor values into Equation 12 can quickly
dominate the total equivalent input referred noise. A 105Ω
source impedance on the non-inverting input will add a
thermal voltage noise term equal to that of the amplifier
itself. As a simplifying constraint, set R_G = R_S in Equation
12 and assume an R_S/2 source impedance at the non-
inverting input (where R_S is the signal’s source impedance
with another matching R_S to ground on the non-inverting
input). This results in Equation 13, where NG > 10 has been
assumed to further simplify the expression.
E_N =
(E_NI )² +
5
4
(I_BR_S
)²
+
4kT


3R_S
2


Eq. 13
Evaluating this expression for R_S = 50Ω will give a total
equivalent input noise of 1.7nV/√Hz. Note that the NG has
dropped out of this expression. This is valid only for NG
> 10.
FREQUENCY RESPONSE CONTROL
Voltage feedback op amps exhibit decreasing closed-loop
bandwidth as the signal gain is increased. In theory, this
relationship is described by the GBP shown in the specifi-
cations. Ideally, dividing GBP by the non-inverting signal
gain (also called the Noise Gain, or NG) will predict the
closed-loop bandwidth. In practice, this only holds true
when the phase margin approaches 90°, as it does in high
gain configurations. At low gains (increased feedback fac-
tor), most high speed amplifiers will exhibit a more complex
response with lower phase margin. The OPA2686 is com-
pensated to give a maximally flat 2nd-order Butterworth
closed-loop response at a non-inverting gain of +10 (Figure
1). This results in a typical gain of +10 bandwidth of
250MHz, far exceeding that predicted by dividing the
1600MHz GBP by 10. Increasing the gain will cause the
phase margin to approach 90° and the bandwidth to more
closely approach the predicted value of (GBP/NG). At a
gain of +40, the OPA2686 will show the 40MHz bandwidth
predicted using the simple formula and the typical GBP of
1600MHz.
Inverting operation offers some interesting opportunities to
increase the available GBP. When the source impedance is
matched by the gain resistor (Figure 2), the signal gain is
(1+R_F/R_G) while the noise gain for bandwidth purposes is
(1 + R_F/2R_G). This cuts the noise gain almost in half,
increasing the minimum stable gain for inverting operation
under these condition to –12 and the equivalent GBP to
3.2GHz.
DRIVING CAPACITIVE LOADS
One of the most demanding and yet very common load
conditions for an op amp is capacitive loading. Often, the
capacitive load is the input of an A/D converter, including
additional external capacitance which may be recommended
to improve A/D linearity. A high speed, high open-loop gain
amplifier like the OPA2686 can be very susceptible to
decreased stability and closed-loop response peaking when
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OPA2686