A true-differential, power-source-free, high-input-impedance amplifier with bipolar output would present distinct advantages in remote devices. Such an amplifier, with its bipolar output, would be a better choice than a unipolar, 4- to 20-mA device. It would also improve on common-mode performance. In Figure 1, a Maxim MAX319 analog switch, IC2, feeds the power from the coaxial signal cable to the charge-holding capacitors, C1 and C2. The analog switch injects both positive- and negative-polarity signals into the charging circuit when its CONTROL signal has a high (TTL) level. At the same time, the output uses a sample-and-hold capacitor to retain the last analog signal during the charging cycle. Thus, the circuit never loses the signal, as long as the charging and sensing cycles maintain timing within certain limits.
|Figure 1.||A high-impedance differential amplifier is useful in remote locations, because it requires no local power supply.|
You can increase the values of C1 and C2 if the sensing time is considerably greater than the charging time. Switch S1 places the feedback resistor, R4, either in a direct connection to the sample-and-hold capacitor, C5, or before R5 to form an R5 -C5 lowpass filter. In either case, R5 is a protective resistor during the charging period, ensuring 10-kΩ load resistance for IC1 a low-bias-current, low-power MAX7614 amplifier. This amplifier is an improved version of the ICL7611. This amplifier circuit was useful for thermocouple-signal amplification without cold-junction correction. You can also use it for bipolar, low-current signal amplification with a range of ±10 pA to ±1 nA, using only R4 for the current-to-voltage conversion.