OPA3680 Datasheet, PDF (18/22 Pages) Burr-Brown (TI) – Triple, Wideband, Voltage-Feedback OPERATIONAL AMPLIFIER With Disable

The total output spot noise voltage can be computed as the
square root of the sum of all squared output noise voltage
contributors. Equation 4 shows the general form for the
output noise voltage using the terms shown in Figure 11.
(4)
( ) ^{( ) ( ) E}O ^{= E}NI2 ^{+ I}BN^RS ^{2 + 4kTR}S ^{NG2 + I}BI^RF ^{2 + 4kTR}F^NG
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 5:
(5)
( ) E_N =
E_NI2 +
I _BN R _S
2
+
4kTR_S
+


I _BI R _F
NG


2
+
4 kTR _F
NG
Evaluating these two equations for the OPA3680 circuit and
component values shown in Figure 1 will give a total output
spot noise voltage of 11nV/√Hz and a total equivalent input
spot noise voltage of 5.5nV/√Hz. This is including the noise
added by the bias current cancellation resistor (100Ω) on the
non-inverting input. This total input-referred spot noise
voltage is only slightly higher than the 4.8nV/√Hz specifica-
tion for the op amp voltage noise alone. This will be the case
as long as the impedances appearing at each op amp input
are limited to the previously recommend maximum value of
125Ω. Keeping both (R_F || R_G) and the non-inverting input
source impedance less than 125Ω will satisfy both noise and
frequency response flatness considerations. Since the resis-
tor-induced noise is relatively negligible, additional capaci-
tive decoupling across the bias current cancellation resistor
(R_B) for the inverting op amp configuration of Figure 9 is not
required.
DC ACCURACY AND OFFSET CONTROL
The balanced input stage of a wideband voltage feedback op
amp allows good output DC accuracy in a wide variety of
applications. The power supply current trim for the OPA3680
gives even tighter control than comparable products. Al-
though the high speed input stage does require relatively
high input bias current (typically 14µA out of each input
terminal), the close matching between them may be used to
reduce the output DC error caused by this current. The total
output offset voltage may be considerably reduced by match-
ing the DC source resistances appearing at the two inputs.
This reduces the output DC error due to the input bias
currents to the offset current times the feedback resistor.
Evaluating the configuration of Figure 1, using worst-case
+25°C input offset voltage and current specifications, gives
a worst-case output offset voltage equal to: – (NG = non-
inverting signal gain)
±(NG • V_OS(MAX)) ± (R_F • I_OS(MAX))
(6)
= ±(2 • 4.5mV) ± (250Ω • 0.7µA)
= ±9.2mV
A fine scale output offset null, or DC operating point
adjustment, is often required. Numerous techniques are
available for introducing DC offset control into an op amp
circuit. Most of these techniques eventually reduce to adding
a DC current through the feedback resistor. In selecting an
offset trim method, one key consideration is the impact on
the desired signal path frequency response. If the signal path
is intended to be non-inverting, the offset control is best
applied as an inverting summing signal to avoid interaction
with the signal source. If the signal path is intended to be
inverting, applying the offset control to the non-inverting
input may be considered. However, the DC offset voltage on
the summing junction will set up a DC current back into the
source which must be considered. Applying an offset adjust-
ment to the inverting op amp input can change the noise gain
and frequency response flatness. For a DC-coupled inverting
amplifier, Figure 12 shows one example of an offset adjust-
ment technique that has minimal impact on the signal fre-
quency response. In this case, the DC offsetting current is
brought into the inverting input node through resistor values
that are much larger than the signal path resistors. This will
insure that the adjustment circuit has minimal effect on the
loop gain and hence the frequency response.
0.1µF
328Ω
+5V
Supply Decoupling
Not Shown
1/3
OPA3680
V_O
+5V
5kΩ
10kΩ
5kΩ
R_G
125Ω
V_I
1.25kΩ
0.1µF
–5V
–5V
R_F
250Ω
±200mV Output Adjustment
V_{O = –} R_{F = –2}
V_I
R_G
FIGURE 12. DC-Coupled, Inverting Gain of –2 with Offset
Adjustment.
®
OPA3680
18