Bipolar current source maintains high output impedance at high frequencies

Conventional current sources and voltage/current converters based on instrumentation and op-amps provide very high output impedance at low frequencies because the amplifiers have good low-frequency CMRR (Common Mode Rejection Ratio). At higher frequencies, reduced CMRR, inherent output capacitance, and slew rate limitations prevent the implementation of high-quality current sources. Two 200 MHz line receiver/amplification ICs from Analog Devices—the AD8129 and AD8130—offer differential inputs and significant CMRR, making them strong candidates for building high-frequency constant current sources. The circuit in Figure 1 provides a good starting point, but the AD8130's higher input bias current may affect output current accuracy at lower current levels.

　　To overcome this problem, a unity gain buffer IC ₂ can be added to isolate the current sensing resistor (Figure 2). Alternatively, a buffer amplifier can be used to measure the load voltage and bootstrap the capacitance of the output cable. This circuit exhibits an output impedance of about 500k at 1MHz, and the current range is 0V to ±3V when using a ±5V supply.

　　Current sources with capacitively coupled loads benefit from a DC servo loop to stabilize the circuit's operating point (Figure 3). The value of the output coupling capacitor CO _depends on the required low frequency attenuation characteristics. Further improvements to the basic circuit enabled compensation of the output capacitance and increased the circuit's output impedance. A small adjustable feedback capacitor, C _COMP , approximately half the stray capacitance at the output, provides feedforward compensation and further reduces the effect of stray capacitance at the output (Figure 4). To prevent oscillations, the cable's shield driver circuit should have a gain slightly less than unity gain. Note: If the output current sensing resistor, R9 _, is reduced to 100Ω, the input attenuator formed by R1 and R2 is compensated and the 1 mA/V _{characteristic} is _maintained . The frequency range of this voltage/current source is 20Hz to 10 MHz. For best results, use high-frequency circuit layout and power supply bypassing methods.