Datasheet

OPA684
14
SBOS219D
www.ti.com
INVERTING SUMMING APPLICATIONS
The OPA684 provides one of the most robust summing
operations available in a wideband op amp. Figure 6 shows
a typical inverting summing application where, in this case, 4
sources are summed through 500Ω gain resistors while also
including an 88.9Ω terminating impedance to present a 75Ω
input impedance to each source. The gain for each channel
is –2 to the output pin and –1 to the matched load. The
extremely low inverting input impedance of the CFB
plus
architectures ensures noninteractive summing for all of the
channels. The amplifier bandwidth is largely independent of
variations in the gain setting elements, depending primarily
on the feedback resistor value instead. This type of circuit
may be used to sum numerous signals together or, where the
prior stages can be disabled, allow multiple channels to be
brought together with only the active channel passing on to
the output.
1kΩ
500Ω
OPA684
+5V
DIS
–5V
IN
2
500Ω
IN
3
500Ω
IN
1
75Ω
500Ω
IN
4
88.9Ω
88.9Ω
88.9Ω
88.9Ω
75Ω Load
Supply Decoupling not shown.
–2(IN
1
+ IN
2
+ IN
3
+ IN
4
)
FIGURE 7. Single-Supply, High-Gain SAW Post-Amplifier.
SAW FILTER POST- AMPLIFIER
While SAW filters provide unmatched selectivity in a passive
filter element, this comes at the cost of significant insertion loss
for the desired signal. The OPA684 can provide a low-cost and
low-power post-amplifier over a wide range of gains due to the
CFB
plus
internal architecture. Figure 7 shows a 44MHz SAW
filter with 20dB insertion loss followed by an OPA684 operating
at a gain of 20V/V (26dB) providing a net gain of 1 (0dB) from
the input of the SAW filter to a matched 50Ω load. The circuit
shown also includes a simple bandpass around 44MHz to
reduce out-of-band noise. The gain for the circuit of Figure 7
ranges from 0dB at DC to 28dB at 44MHz and then back to
< 0dB above 200MHz. The LC elements in the gain circuit peak
the response slightly above a gain of 26dB—removing those
would give a broadband gain of 26dB.
950Ω
OPA684
+V
S
DIS
50Ω
50Ω Load
20kΩ
20kΩ
50Ω
1000pF
1000pF
100pF
1000pF
160nH
P
I
P
I
P
O
P
O
50Ω
SAW
Filter
44MHz
20dB
Insertion
Loss
+5V to +12V
= 0dB at 44MHz
FIGURE 6. Gain of –2 Video Summing Amplifier.