The I/V-converter

1. Introduction An I/V-convertor is intended to convert the current from a DAC (Digital Analog Convertor) into an analog voltage and to filter this signal to keep out the alias frequencies. Often these functions are separated into two stages: one for the conversion, and another for the low pass filtering. I will call them: 'the I/VC' and 'the lp-filter'. The current from the DAC (often 2.5 mA) is far from clean. Outside the audio frequency domain the aliases and rest-signals of digital origin with a wide frequency spectrum do exist. The input impedance of the I/VC should be constant and low (< 5 ohm) over the whole frequency range in question. Both stages should perform without noticeable distortion (< 0.001 %). 2. The I/VC What about the high frequency components in the DAC-current? Cannot they be filtered out before entering high open loop gain stages? Cannot high open loop gain stages be avoided at all? Because of the low distortion figures and a reasonable low output impedance, at least the lp-filter does need feedback. Because of the prescribed low input impedance of the I/VC, it must start with an emitter- or source-input stage if no use is made of 'the virtual ground-circuit' with an op amp. 2.1   I/VC with discrete bipolars The discrete I/VC-circuit with bipolar transistors (at the right) performs best if the second harmonics have been cancelled with the proper value of R1 and R2. The third harmonic however is still .04 %. (R5 is used to get a first idea of the input impedance in the plots.) The frequency response is excellent. The slope is even 12 dB/oct due to the extra 56 nF. R6 cannot be much higher than 100 ohm on penalty of more distortion. The output voltage is 0.25 volt (with a DAC-current of 2.5 mA) so that appending amplification of 20x is needed. 2.2  I/VC with a FET The discrete I/VC-circuit with ten times the FET J310 in parallel in GGS performs very well. The frequency response is excellent and the distortion is low. The circuit shown at the right is only for simula- tion purposes. Of course in practice the DC-level of the input should be 0 volt. This will be realised by biasing the gate. As with the discrete bipolar transistors, some measures should be taken to get a better power supply rejection (PSR). The power consumption (some 70 mA) is high to serve a low source-impedance. Instead of the ten J310's, one could look after a large FET (with cooling!). A current transformer is another solution if the 'leakage induction' could be kept low enough. With one J310 a step up of 1 : 3 will satisfy. Apart from the difficulties constructing a good transformer (could an old MC-cartridge transformer be of help?), a transformer is sensitive to magnetic fields so at least with the mains transformer in the same box, this solution offers hum.

The lp-filter eventually has to actuate the following audio system with about 3 volt at 0 dB. Often op amps are used in both stages. At least the usage of an op amp in the I/VC is criticized: 'How could one put such a complex signal in a circuit with a vary high open loop gain?' It has to be clear that an op amp in this application must have a large GB-product. The (12 dB/octave) slope of the lp-filter should start at 20 kHz and drop monotonic to -160 dB! Before the circuits are put together, they will be simulated with MicroSim8, a more then ten years old SW-package on PC which satisfies. These simulations are presented here.