TI op

TI has combined a modified chopper architecture with a proprietary input to create an op-amp whose precision extends to common-mode signals anywhere within its power rails – and slightly beyond.

Traditional rail-to-rail inputs use a pair of differential pairs, one p-channel (or pnp) and one n-channel (or npn), whose outputs are combined in a later stage (see diagram).

Near the negative rail, the pnp differential pair operates, and neat the positive rail the npn pair operates. Somewhere in between, one differential pair smoothly hands over to the other.

At these extreme, input offset voltage is easily controlled, but in the common-mode range where both are partially operating it is far harder to maintain offset control – leading to cross-over distortion, according to TI.

To avoid the crossover issue, TI has re-used a earlier technology from its portfolio – a single p-channel differential pair combined with a charge-pump which raises the sources of the pair high enough to bring the positive rail (and 100mV beyond) into common-mode range (see more).

This input is combined with a chopper amplifier – which TI is calling a ‘zero-drift’ – where connections to its differential inputs are swapped by switches (shown as crossed boxes in the diagram) controlled by a clock. At the same time, the differential outputs are swapped by the same clock, synchronously rectifying the signal to reconstruct an amplified version of the input signal. Any DC offsets are mixed away from dc up to the clock frequency (see more).

"TI's zero-drift technology eliminates temperature drift and flicker noise to attain the highest dc precision and dynamic error correction, while its zero-crossover topology eliminates offset errors caused by common-mode limitations to achieve linear output and true rail-to-rail input operation," said the firm.

The result is the OPA388, which is specified at 5µV maximum input offset and typical offset drift of 5nV/°C. Maximum input bias is 700pA over -40 to 125°C.

Gain bandwidth product is 10MHz (148dB open-loop gain) making it "possible to acquire a wide range of signal types and frequencies to support equipment from precision weigh scales to heart-rate monitors", said TI.

Ac parameters are -132dBc total harmonic distortion and 7nV/√Hz noise which "help produce a high-resolution signal chain for specialised applications such as programmable logic controllers, precision field transmitters and motion-control equipment."

A reference design is available using the op-amp to eliminate cross-over non-linearity in DACs – using the precision DAC8830 and REF5050 voltage reference.

Operation is over 2.5 to 5.5V (±1.25 to ±2.75V) and supply current is typically 1.7mAand 2.4mA max.

It is available in a 4.9 x 3.9mm SOIC, and both PSPICE and TINA-TI models are available.

Applications are foreseen in test, measurement, medical, safety equipment, and high-resolution data-acquisition.