Selectable trimming circuit for use with a digital to analog converter

In an integrated circuit digital-to-analog signal converter comprising a plurality of stages each connected to a converter output terminal, each stage including a current source-connected transistor and a switch means connected in circuit with said transistor, each of said transistors being contructed to provide a predetermined current magnitude within a given tolerance range, each of said switch means being controlled by an input digital signal to switch its respective transistor with respect to said output terminal such that the total current delivered to said terminal from said stages is representative of the input digital signal, wherein the improvement comprises:

• a) a plurality of trim circuits associated respectively with at least some of said stages, each of said trim circuits comprising:
• (1) a plurality of transistors adapted to receive a supply voltage and connected to a common trim output terminal, the physical dimensions of each of said transistors being scaled in proportion to a desired level of current flow for each transistor, said trim terminal being connected to provide trimming current to its associated converter stage,
• (2) selectable switch means connected in circuit with each of said transistors for enabling current flow through selected ones of said transistors,
• (3) means for transmitting actuating signals to permanently actuate selected ones of said switch means irrespective of changes in the supply voltage,
• (4) each of said transistor-switch circuits being adpated to receive a supply voltage sufficient to establish a desired current flow through its transistor when its associated switch means is actuated, whereby a permanent desired level of corrective trimming current may be attained at each trim output terminal by accumulating the current flows through its associated selected transistors, and
• (5) means for combining the output current of each trimmed stage with its respective trimming current to produce a permanently corrected output current for said stage,
• (b) means for establishing a common reference current, and
• (c) proportional current mirror means associated with each of said trim circuits,
• (d) each of said current mirror means mirroring a proportion of said common reference current to provide an individual reference current for its associated trim circuit, the magnitude of said individual reference currents being in mutual non-binary relationship, and the proportion for each trim circuit being selected in proportion to the relative error tolerance of its associated stage.

In an integrated circuit digital-to-analog signal converter comprising a plurality of stages each connected to a converter output terminal, each stage including a current source-connected transistor and a switch means connected in circuit with said transistor, each of said transistors being constructed to provide a predetermined current magnitude within a given tolerance range, each of said switch means being controlled by an input digital signal to switch its respective transistor with respect to said output terminal such that the total current delivered to said terminal from said stages is representative of the input digital signal, wherein the improvement comprises a plurality of trim circuits associated respectively with at least some of said stages, each of said trim circuits comprising:

• a plurality of transistors adapted to be coupled with a voltage supply and connected to a common trim output terminal, the physical dimensions of each of said transistors being scaled in proportion to a desired level of current flow for each transistor, said trim terminal being connected to provide trimming current to its associated converter stage,
• selectable switch means connected in circuit with each of said transistors for enabling current flow through selected ones of said transistors,
• means for transmitting actuating signals to permanently actuate selected ones of said switch means irrespective of changes in the supply voltage,
• each of said transistor-switch circuits being adapted to receive a supply voltage sufficient to establish a desired current flow through its transistor when its associated switch means is actuated, whereby a permanent desired level of corrective trimming current may be attained at each trim output terminal by accumulating the current flows through its associated selected transistors,
• means for combining the output current of each trimmed stage with its respective trimming current to produce a permanently corrected output current for said stage,
• means for establishing a common reference current,
• a plurality of proportional current mirror means each mirroring a proportion of said common reference current to provide individual reference currents for respective ones of said trim circuits, the proportion of current mirrored by each of said proportional current mirror means being determined in proportion to the relative error tolerance of its associated converter stage, and in non-binary relationship to the reference currents for the other stages, and
• means for permanently adjusting the level of said common reference current, and thereby enabling the production of high resolution converters from relatively accurate untrimmed converters by the selection of a relatively small common reference current, while also enabling a greater trimming range for converters with a relatively lower untrimmed accuracy by the selection of a relatively larger common reference current,
• said means for permanently adjusting the common reference current including a first transistor connected to support an auxiliary reference current, a plurality of parallel connected transistors mirroring said first transistor to pass currents the magnitudes of which are substantially fixed proportions of said auxiliary reference current, and permanently selectable switch means connected in circuit with at least some of said parallel connected transistors for controlling the current flow therethrough, said parallel transistor currents being accumulated to provide said common reference current, and
• said permanently selectable switch means comprising a plurality of zener diodes, each of said diodes being connected in series circuit with a respective one of said trim circuit transistors to block current flow through said transistor, and means for transmitting a breakdown voltage differential to selected ones of said diodes to enable current flow through their associated transistors.