Sequential PROPHET-600 - Page 85

at the V pin should be kept less than 50K to minimize control
input loading errors. Typical external connections for a
linear response is shown in Figures 1 and 2.
Figure 3 shows the typical response of gain versus control
voltage for the linear control input. As can be seen, this
response curve is not perfectly linear, but has a rounded knee
at the very beginning and at the very end. In addition, for
negative supply voltages less than -12 volts, there is a small
positive threshold which must be overcome before the cell gain
begins to increase at all; at -3 volts, this threshold is
tyoically +180 mV. Thresholds at other negative supply voltages
are given in Figure 4. If an application at negative supplies
less than -12 volts requires this turn-on threshold to be
minimized, than a positive offset equal to the minimum threshold
shown in Figure 4 can be built into the control circuit. In
the circuits of Figures 1 and 2, for instance, this offset
can be added simply with a resistor from the V c pin to the
positive supply voltage.
To provide an exponential control response, the log converter
is not used at all (the V and the V pins may be simply loft
open) and the control voltage is applied directly to the V
pin. In Figure 5 is shown the typical response of cell cuFrent
gain in dB versus the voltage on pin V_ with respect to the
reference voltage, V__ p , on pin 8. As can be seen, the exponential
conformity is very gStSfl from -90dB to -20dB, corresponding to
a control voltage of 270mV to 60mV below the reference voltage,
nominally 1.8V. For the last 20dB of attenuation, corresponding
to -60mV to +60mV with respect to V REF , the transfer curve is quite
rounded.
A typical circuit connection for an exponential control response
is shown in Figure 6. For best gain consistency from device
to device, the input resistor divider is referenced to the
reference voltage on pin 8. The resistor ratio is selected
such that with a control voltage of zero, the voltage appearing
on the V pin is 240 mV below V^, and the corresponding
current gain is -80dB.
On the linear portion of the curve, the gain will increase
20dB for approximately every 450mV increase in control voltage;
maximum gain (0 dB) will occur at approximately +2.2 volts. A
wider control input range may be accommodated by adding a resistor
from the V pin to the negative supply and ratioing the resistors
so that thl input voltage range produces the desired voltage
excursion on the V E pin.
The impedance of the resistor network at the pin should be
keot below 5K to minimize loading errors by the V pin. Also,
care should be exercised to ensure that the resistor networks
connected to pin 8 do not source more than a total of 400uA
or sink more than a total of 1mA from this pin.
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