Datasheet

OPA699
16
SBOS261D
www.ti.com
WIDEBAND INVERTING OPERATION
Operating the OPA699 as an inverting amplifier has several
benefits and is particularly useful when a matched 50Ω
source and input impedance are required. Figure 3 shows
the inverting gain of –4V/V circuit used as the basis of the
inverting mode typical characteristics.
LOW-GAIN COMPENSATION FOR IMPROVED SFDR
Where a low gain is desired, and inverting operation is
acceptable, a new external compensation technique can be
used to retain the full slew rate and noise benefits of the
OPA699, while giving increased loop gain and the associ-
ated distortion improvements offered by a non-unity-gain
stable op amp. This technique shapes the loop gain for good
stability, while giving an easily controlled 2nd-order low-pass
frequency response. To set the compensation capacitors (C
S
and C
F
), consider the half-circuit of Figure 5, where the 50Ω
source is used.
Considering only the noise gain for the circuit of Figure 5, the
low-frequency noise gain (N
G1
) is set by the resistor ratio,
while the high-frequency noise gain (N
G2
) is set by the
capacitor ratio. The capacitor values set both the transition
frequencies and the high-frequency noise gain. If the high-
frequency noise gain, determined by N
G2
= 1 + C
S
/C
F
, is set
to a value greater than the recommended minimum stable
gain for the op amp, and the noise gain pole (set by 1/R
F
C
F
)
is placed correctly, a very well controlled 2nd-order low-pass
frequency response results.
In the inverting case, only the feedback resistor appears as
part of the total output load in parallel with the actual load. For
a 500Ω load used in the typical characteristics, this gives a
total load of 329Ω in this inverting configuration. The gain
resistor is set to get the desired gain (in this case, 187Ω for
a gain of –4) while an additional input resistor (R
M
) can be
used to set the total input impedance equal to the source, if
desired. In this case, R
M
= 68.1Ω in parallel with the 187Ω
gain setting resistor gives a matched input impedance of
50Ω. This matching is only needed when the input needs to
be matched to a source impedance, as in the characteriza-
tion testing done using the circuit of Figure 3.
For bias current-cancellation matching, the noninverting input
requires a 169Ω resistor to ground. The calculation for this
resistor includes a DC-coupled 50Ω source impedance along
with R
G
and R
M
. Although this resistor will provide cancella-
tion for the bias current, it must be well-decoupled (0.1µF in
Figure 3) to filter the noise contribution of the resistor and the
input current noise.
As the required R
G
resistor approaches 50Ω at higher gains,
the bandwidth for the circuit in Figure 3 will far exceed the
bandwidth at that same gain magnitude for the noninverting
circuit of Figure 1. This occurs due to the lower noise gain for
the circuit of Figure 3 when the 50Ω source impedance is
included in the analysis. For instance, at a signal gain of –15
(R
G
= 50Ω, R
M
= open, R
F
= 750Ω) the noise gain for the
circuit of Figure 3 will be 1 + 750Ω/(50Ω + 50Ω) = 8.5 due to
the addition of the 50Ω source in the noise gain equation.
This approach gives considerably higher bandwidth than the
noninverting gain of +15. Using the 1GHz gain bandwidth
product for the OPA699, an inverting gain of –15 from a 50Ω
source to a 50Ω R
G
will give 140MHz bandwidth, whereas
the noninverting gain of +8 will give 55MHz, as shown in the
measured results of Figure 4.
OPA699
–5V
V
I
–2V
+5V +2V
R
M
68.1Ω
R
F
750Ω
R
G
187Ω
500Ω
0.1µF
R
T
169Ω
V
H
V
L
V
O
50Ω Source
1M 10M 100M 1G
Frequency (Hz)
Gain (dB)
24
21
18
15
12
9
G = +15
G = –15
R
F
402Ω
C
S
13pF
OPA699
+5V
–5V
V
O
V
I
C
F
2.8pF
200Ω
R
G
402Ω
V
H
V
L
FIGURE 3. Inverting G = –4 Specifications and Test Circuit.
FIGURE 4. G = +15 and –15 Frequency Response.
FIGURE 5. Broadband, Low-Inverting Gain External
Compensation.