Datasheet

R =462 NGW -
F I
· R
450
400
350
300
250
200
150
NoiseGain
0 2010 155
FeedbackResistor( )W
OPA694
SBOS319G –SEPTEMBER 2004–REVISED JANUARY 2010 www.ti.com
gain reduces to 1 (and the curves intersect). This bandwidth. Inserting a series resistor between the
point of equality is where the amplifier closed-loop inverting input and the summing junction will increase
frequency response given by Equation 1 starts to roll the feedback impedance (denominator of Equation 1),
off, and is exactly analogous to the frequency at decreasing the bandwidth. This approach to
which the noise gain equals the open-loop voltage bandwidth control is used for the inverting summing
gain for a voltage-feedback op amp. The difference circuit on the front page. The internal buffer output
here is that the total impedance in the denominator of impedance for the OPA694 is slightly influenced by
Equation 2 may be controlled somewhat separately the source impedance looking out of the noninverting
from the desired signal gain (or NG). input terminal. High source resistors will have the
effect of increasing R
I
, decreasing the bandwidth.
The OPA694 is internally compensated to give a
maximally flat frequency response for R
F
= 402Ω at
OUTPUT CURRENT AND VOLTAGE
NG = 2 on ±5V supplies. Evaluating the denominator
of Equation 2 (which is the feedback transimpedance)
The OPA694 provides output voltage and current
gives an optimal target of 462Ω. As the signal gain
capabilities that are not usually found in wideband
changes, the contribution of the NG × R
I
term in the
amplifiers. Under no-load conditions at +25°C, the
feedback transimpedance will change, but the total
output voltage typically swings closer than 1.2V to
can be held constant by adjusting R
F
. Equation 3
either supply rail; the +25°C swing limit is within 1.2V
gives an approximate equation for optimum R
F
over
of either rail. Into a 15Ω load (the minimum tested
signal gain:
load), it is tested to deliver more than ±60mA.
(3)
The specifications described above, though familiar in
the industry, consider voltage and current limits
As the desired signal gain increases, this equation
separately. In many applications, it is the (voltage ×
will eventually predict a negative R
F
. A somewhat
current), or V-I product, which is more relevant to
subjective limit to this adjustment can also be set by
circuit operation. Refer to the Output Voltage and
holding R
G
to a minimum value of 20Ω. Lower values
Current Limitations plot (Figure 21) in the Typical
will load both the buffer stage at the input and the
Characteristics. The X and Y axes of this graph show
output stage, if R
F
gets too low, actually decreasing
the zero-voltage output current limit and the
the bandwidth. Figure 39 shows the recommended
zero-current output voltage limit, respectively. The
R
F
versus NG for ±5V operation. The values for R
F
four quadrants give a more detailed view of the
versus gain shown here are approximately equal to
OPA694 output drive capabilities, noting that the
the values used to generate the Typical
graph is bounded by a Safe Operating Area of 1W
Characteristics. They differ in that the optimized
maximum internal power dissipation. Superimposing
values used in the Typical Characteristics are also
resistor load lines onto the plot shows that the
correcting for board parasitics not considered in the
OPA694 can drive ±2.5V into 25Ω or ±3.5V into 50Ω
simplified analysis leading to Equation 2. The values
without exceeding the output capabilities or the 1W
shown in Figure 39 give a good starting point for
dissipation limit. A 100Ω load line (the standard test
design where bandwidth optimization is desired.
circuit load) shows the full ±3.4V output swing
capability, as shown in the Electrical Characteristics.
The minimum specified output voltage and current
over-temperature are set by worst-case simulations at
the cold temperature extreme. Only at cold startup
will the output current and voltage decrease to the
numbers shown in the Electrical Characteristic tables.
As the output transistors deliver power, the junction
temperatures will increase, decreasing both V
BE
(increasing the available output voltage swing) and
increasing the current gains (increasing the available
output current). In steady-state operation, the
available output voltage and current will always be
greater than that shown in the over-temperature
specifications, since the output stage junction
temperatures will be higher than the minimum
specified operating ambient.
Figure 39. Feedback Resistor vs Noise Gain
The total impedance going into the inverting input
may be used to adjust the closed-loop signal
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