A device for computing a function value of a function F, comprising: a memory containing a look-up table comprising a set of entries (L1; L2; L3; L4), each of which entries having associated with it a domain (D1; D2; D3; D4) and an approximation function (F1; F2; F3; F4) for approximating the function F on the associated domain; a truncator unit arranged to truncate or round a first value X1 to generate a second value X2; a selector unit arranged to select an entry (L1; L2; L3; L4) of the lookup-table according to the second value X2, thus selecting the approximation function (F1; F2; F3; F4) that is associated with the selected entry; and an evaluator unit arranged to determine the function value of the selected approximation function (F1; F2; F3; F4) at the first value X1; wherein at least one of the entries (L1; L2; L3; L4) of the look-up table comprises at least one number in fixed point format and the device comprises a fixed-point to floating-point converter for converting said number to floating-point format, or wherein at least one of the entries (L1; L2; L3; L4) of the look-up table comprises at least one number in floating point format and the device comprises a floating-point to fixed-point converter for converting said number to fixed-point format; wherein the fixed-point to floating-point converter and/or the floating-point to fixed-point converter comprises an input unit for receiving an input number; a set of at least two channels (26; 28; 30), each of which channels is configured for generating an output number (27; 29; 31) from the input number by a channel-specific method; and a multiplexer for selecting one of the generated output numbers according to an index indicative of the selected entry (L1; L2; L3; L4) and/or according to one or more characteristics of the input number. a memory containing a look-up table comprising a set of entries (L1; L2; L3; L4), each of which entries having associated with it a domain (D1; D2; D3; D4) and an approximation function (F1; F2; F3; F4) for approximating the function F on the associated domain; a truncator unit arranged to truncate or round a first value X1 to generate a second value X2; a selector unit arranged to select an entry (L1; L2; L3; L4) of the lookup-table according to the second value X2, thus selecting the approximation function (F1; F2; F3; F4) that is associated with the selected entry; and an evaluator unit arranged to determine the function value of the selected approximation function (F1; F2; F3; F4) at the first value X1; wherein at least one of the entries (L1; L2; L3; L4) of the look-up table comprises at least one number in fixed point format and the device comprises a fixed-point to floating-point converter for converting said number to floating-point format, or wherein at least one of the entries (L1; L2; L3; L4) of the look-up table comprises at least one number in floating point format and the device comprises a floating-point to fixed-point converter for converting said number to fixed-point format; wherein the fixed-point to floating-point converter and/or the floating-point to fixed-point converter comprises an input unit for receiving an input number; a set of at least two channels (26; 28; 30), each of which channels is configured for generating an output number (27; 29; 31) from the input number by a channel-specific method; and a multiplexer for selecting one of the generated output numbers according to an index indicative of the selected entry (L1; L2; L3; L4) and/or according to one or more characteristics of the input number.