The RFC-1 is will give a full scale telemetry reading with as little as 1.0 volt DC. This is adequate sensitivity for the vast majority of telemetry sources common in broadcasting. In some cases the available telemetry voltage is too low and a DC amplifier is necessary. To insure accurate telemetry readings, the DC amplifier must have highly stable gain characteristics and contribute negligible DC offset.
The circuit below has a DC gain of 10 with the required level of accuracy. The integrated circuit is an LT1101 instrumentation amplifier. It has a differential input with a very high impedance. It can be powered by a simple, unregulated power supply delivering +10 to +25 volts.
Quantity
Digi-Key Part #
Part Description
1
LT1101CN8
Op amp IC
1
AE9808
8 pin DIP socket
4
P4923
0.1mF 50V monolithic ceramic capacitor
1
1.5KQBK
1.5K Ohm resistor
1
27KQBK
27K Ohm resistor
1
T509
12 VDC power supply
Digi-Key parts can be located here
How to build a DC isolation amplifier for the RFC-1/B
In some cases, a high degree of isolation may be required but DC gain is not needed. This can be accomplished by setting the gain of the amplifier to 100 and adding a 100:1 attenuator to the input. The circuit below has an input impedance of approximately 5M ohms with no DC gain (unity).
Quantity
Digi-Key Part #
Part Description
1
LT1101CN8
Op amp IC
1
AE9808
8 pin DIP socket
4
P4923
0.1mF 50V monolithic ceramic capacitor
1
1.5KQBK
1.5K Ohm resistor
1
47KQBK
47K Ohm resistor
1
2.4MQBK
2.4M Ohm resistor
1
T509
12 VDC power supply
