OPA687 Burr-Brown, OPA687 Datasheet - Page 10

• Manufacturer Part Number: OPA687
• Description: Wideband / Ultra-Low Noise / Voltage Feedback OPERATIONAL AMPLIFIER With Power Down
• Manufacturer: Burr-Brown

signal. If the source is differential already, it may be con-
nected through blocking capacitors into the gain setting
resistors. To set the compensation capacitors for this circuit
(C
50 source is reflected through the 1:2 transformer and then
cut in 1/2 and grounded to give a total impedance to AC
ground (for the circuit on the front page of this data sheet)
equal to the 200 .
Considering only the noise gain (this is the same as the non-
inverting signal gain) for the circuit of Figure 4, the low
frequency noise gain, (NG
while the high frequency noise gain (NG
capacitor ratios. The capacitor values set both the transition
frequencies and the high frequency noise gain. If the high
frequency noise gain, determined by NG
to a value greater than the recommended minimum stable
gain for the op amp, and the noise gain pole, set by 1/R
is placed correctly, a very well-controlled, second-order low
pass frequency response will result.
To choose the values for both C
only three equations need to be solved. The first parameter
is the target high frequency noise gain NG
greater than the minimum stable gain for the OPA687. Here,
a target NG
desired low frequency signal gain, which also sets the low
frequency noise gain NG
will target a maximally flat second-order low pass Butterworth
frequency response (Q = 0.707). The signal gain of –4.25
shown in Figure 4 will set the low frequency noise gain to
NG
these two gains and the GBP for the OPA687 (3600MHz),
the key frequency in the compensation can be determined as:
FIGURE 4. Broadband Low Inverting Gain External Com-
S
1
and C
V
= 1 + R
I
Z
O
F
), consider the 1/2 circuit of Figure 4 where the
2
200
pensation.
F
R
of 24 will be used. The second parameter is the
NG
®
/R
GBP
G
G
OPA687
1
(= 5.25 in this example). Then, using only
2
1 –
44pF
C
S
NG
1
NG
1
. To simplify this discussion, we
) will be set by the resistor ratios
OPA687
+5V
–5V
1
2
S
– 1 – 2
and C
850
1.9pF
R
C
F
F
F
, two parameters and
2
2
NG
NG
) will be set by the
= 1 + C
2
, which should be
1
2
S
/C
V
O
F
, is set
F
C
F
,
10
Physically, this Z
set by 1/(2 • R
rising portion of the noise gain would intersect unity gain if
projected back to 0dB gain. The actual zero in the noise gain
occurs at NG
NG
and use this to get C
Finally, since C
determine C
The resulting closed-loop bandwidth will be approximately
equal to:
For the values shown in Figure 4, the f
mately 121MHz. This is less than that predicted by simply
dividing the GBP product by NG
network controls the bandwidth to a lower value while
providing the full slew rate at the output and an excep-
tional distortion performance due to increased loop gain at
frequencies below NG
in Figure 4 are calculated for NG
with no adjustment for parasitics. The full circuit on the
front page of this datasheet shows the capacitors adjusted
for parasitics.
The front page of this data sheet shows the measured 2-tone,
3rd-order distortion for just the amplifier portion of the
circuit.
The upper curve is for a total 2-tone envelope of 4Vp-p,
requiring two tones, each at 2Vp-p across the OPA687
outputs. The lower curve is for a 2Vp-p envelope requiring
each tone to be 1Vp-p. The basic measurement dynamic
range for the two close-in spurious tones is approximately
85dBc. The 4Vp-p test does not show measurable 3rd-order
spurious until 25MHz ,while the 2Vp-p is unmeasurable up
to 40MHz center frequency. Two-tone, 2nd-order
intermodulation distortion was unmeasurable for the circuit
on the front page of this data sheet.
2
• Z
0
. Since GBP is expressed in Hz, multiply Z
f
– dB
S
C
C
1
by [Using NG
S
F
• Z
F
S
(C
0
and C
0
F
NG
(4.1MHz for the values shown above) is
and the pole in the noise gain occurs at
Z
+ C
F
• R
O
2
by solving:
1
F
GBP
S
– 1 C
)) and is the frequency at which the
• Z
F
set the high frequency noise gain,
Z
O
0
2
. The capacitor values shown
NG
= 24]:
F
1
(= 1.90pF)
(= 43.8pF)
(= 121MHz)
= 5.25 and NG
1
. The compensation
–3dB
will be approxi-
0
2
by 2
= 24