Author name: gremblor

I need a minimal-lag zero-cross detector for the tone control section of an audio preamp which will rely on digital potentiometers for adjustment. The pot adjustments (communicated from an MCU over SPI) need to be sync’d to 0V input to avoid popcorn noise.

The digital pot has a write-inhibit input that can be driven low to latch the adjustment while the input signal is crossing 0V. My goal is to hit this pretty close to the mark (±100mV), for any audio-rate signal, which may be up to 3.5 Vrms (~10 Vpp) and 20 kHz. To be valid, the signal needs to remain low for 25ns. I’d like to hit this with room to spare and also minimize signal bounce / glitching for a slowly-changing audio-rate input.

A new IC by Texas Instruments that makes this possible with high resolution and minimal phase lag is the TLV1871.

I developed this automatic gain control (AGC) circuit to compensate for the -3 dB gain after each filter stage in a 2-stage, 4-pole voltage-controlled filter (VCF). I’m posting this because it was surprisingly difficult (rather, impossible) to find a low-distortion operational transconductance amplifier (OTA) driven AGC schematic I could use directly.

In Part 2 of the Precision Current Reference build, I assembled and calibrated the instrument and tested it out.

A precision current source like you’d find in an SMU would be a very helpful tool for some of my other projects. But an SMU costs an arm and a leg… my needs were not so great. Could I make what I needed without breaking the bank?