OPA2694ID Datasheet, PDF (19/28 Pages) Texas Instruments – Dual, Wideband, Low-Power, Current Feedback Operational Amplifier

OPA2694
www.ti.com
alone. This reflects the noise added to the output by the
inverting current noise times the feedback resistor. If the
feedback resistor is reduced in high-gain configurations
(as suggested previously), the total input-referred voltage
noise given by Equation (5) will approach just the
2.1nV/√Hz of the op amp itself. For example, going to a
gain of +10 using R_F = 178Ω will give a total input-referred
noise of 2.36nV/√Hz.
DC ACCURACY AND OFFSET CONTROL
A current-feedback op amp like the OPA2694 provides
exceptional bandwidth in high gains, giving fast pulse
settling, but only moderate DC accuracy. The Electrical
Characteristics show an input offset voltage comparable to
high-speed, voltage-feedback amplifiers. However, the
two input bias currents are somewhat higher and are
unmatched. Whereas bias current cancellation
techniques are very effective with most voltage-feedback
op amps, they do not generally reduce the output DC offset
for wideband, current-feedback op amps. Since the two
input bias currents are unrelated in both magnitude and
polarity, matching the source impedance looking out of
each input to reduce their error contribution to the output
is ineffective. Evaluating the configuration of Figure 1,
using worst-case +25°C input offset voltage and the two
input bias currents, gives a worst-case output offset range
equal to:
± (NG × V_OS) ± (I_BN × R_S/2 × NG) ± (I_BI × R_F)
where NG = noninverting signal gain
= ± (2 × 3.2mV) ± (22μA × 25Ω × 2) ± (402Ω × 20μA)
= ±6.4mV + 1.1mV ± 8.04mV = ±15.54mV
A fine-scale, output offset null, or DC operating point
adjustment, is sometimes required. Numerous techniques
are available for introducing DC offset control into an op
amp circuit. Most simple adjustment techniques do not
correct for temperature drift. It is possible to combine a
lower speed, precision op amp with the OPA2694 to get
the DC accuracy of the precision op amp along with the
signal bandwidth of the OPA2694. Figure 13 shows a
noninverting G = +10 circuit that holds an output offset
voltage less than ±7.5mV over-temperature with
> 150MHz signal bandwidth.
SBOS320D − SEPTEMBER 2004 − REVISED APRIL 2013
Power−supply
+5V
decoupling not shown.
DIS
V_I
1/2
OPA2694
V_O
1.8kΩ
+5V
OPA237
2.86kΩ
−5V _180Ω
20Ω
−5V
18kΩ
2kΩ
Figure 13. Wideband, DC-Connected Composite
Circuit
This DC-coupled circuit provides very high signal
bandwidth using the OPA2694. At lower frequencies, the
output voltage is attenuated by the signal gain and
compared to the original input voltage at the inputs of the
OPA237 (this is a low-cost, precision voltage-feedback op
amp with 1.5MHz gain bandwidth product). If these two do
not agree (due to DC offsets introduced by the OPA2694),
the OPA237 sums in a correction current through the
2.86kΩ inverting summing path. Several design
considerations will allow this circuit to be optimized. First,
the feedback to the OPA237 noninverting input must be
precisely matched to the high-speed signal gain. Making
the 2kΩ resistor to ground an adjustable resistor would
allow the low- and high-frequency gains to be precisely
matched. Second, the crossover frequency region where
the OPA237 passes control to the OPA2694 must occur
with exceptional phase linearity. These two issues reduce
to designing for pole/zero cancellation in the overall
transfer function. Using the 2.86kΩ resistor will nominally
satisfy this requirement for the circuit in Figure 13. Perfect
cancellation over process and temperature is not possible.
However, this initial resistor setting and precise gain
matching will minimize long-term pulse settling tails.
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