Fully programmable bias current generator with 24 bit resolution per bias

This paper describes an on-chip programmable bias current generator, intended for mixed signal chips requiring a wide ranging set of currents. The individual generators share a master current reference. A serial digital interface to the chip controls the biases by bits loaded into a 24-bit shift register. These bits control the steering of current from a current splitter. The summed current splitter output is actively mirrored to a broadcasted bias voltage. Measurements from an implementation in 0.35mu 4M-2P CMOS show a total range of bias current of over 6 decades (>120 dB) ranging from a few times the off-current up to the master reference current. For currents larger than the minimum, the generator has resolution spanning nearly its full 24 bit range (144dB), e.g. for a master current of 10 muA, any bias current can be varied by as little as 0.5 pA with the caveat that the code is not guaranteed monotonic. Each bias occupies an area of 0.026 mm² , which is about 65% of the bonding pad that it replaces. Measured variation in generated currents is <10% in strong inversion and about 20-30% in weak inversion