A/D conversion circuit and test method

1. An A/D conversion circuit, comprising:

a plurality of transmission paths that transmit signal voltages and reference voltages; and

an A/D conversion unit that A/D converts voltages output from the transmission paths, wherein

each of the plurality of transmission paths comprises:

a first switch that selectively outputs one of the signal voltage and the reference voltage;

a hold circuit that holds output voltage from the first switch; and

a second switch that selectively outputs one of the output voltage from the first switch and output voltage from the hold circuit.

2. The A/D conversion circuit according to claim 1 , wherein, in a test operation, the A/D conversion unit A/D converts the output voltage from the first switch input through the second switch without being held in the hold circuit, and thereafter A/D converts the output voltage from the hold circuit input through the second switch.

3. The A/D conversion circuit according to claim 1 , wherein the hold circuit comprises:

a capacitor that holds the output voltage from the first switch; and

a third switch that electrically connects the first switch and the capacitor.

4. The A/D conversion circuit according to claim 1 , further comprising a selection circuit that selectively supplies output voltages from the plurality of transmission paths to the A/D conversion unit.

5. The A/D conversion circuit according to claim 1 , further comprising a digital value hold circuit that holds digital values output from the A/D conversion circuit, the digital values being associated with the plurality of transmission paths.

6. The A/D conversion circuit according to claim 1 , wherein the A/D conversion unit carries out A/D conversion processing on the output voltages of the hold circuits for a plurality of times with time intervals therebetween.

7. The A/D conversion circuit according to claim 1 , further comprising a reference voltage generation circuit that generates the reference voltages,

wherein the reference voltage generation circuit supplies the reference voltages to the first switch, the reference voltages having voltage values that are different from each other.

8. The A/D conversion circuit according to claim 1 , further comprising a control circuit that controls selection states of at least the first and the second switches.

9. A semiconductor device, comprising:

the A/D conversion circuit according to claim 1 ;

a CPU that instructs the A/D conversion circuit to execute A/D conversion; and

a time measurement circuit that measures a time based on the instruction from the CPU and instructs the A/D conversion circuit to execute the A/D conversion after a completion of a measurement of a predetermined time.

10. The A/D conversion circuit according to claim 1 , wherein the A/D conversion unit receives, through the second switch, an output voltage from the first switch and an output voltage from the hold circuit.

11. A test method, comprising:

A/D converting a first reference voltage input through a path while keeping the first reference voltage stored in a hold circuit, the path being arranged to be parallel with the hold circuit;

A/D converting a second reference voltage input through the path while keeping the hold circuit floated; and

canceling a floating state of the hold circuit to A/D convert output voltage from the hold circuit.

12. The test method according to claim 11 , wherein the first reference voltage and the second reference voltage have voltage values that are different from each other.

13. The test method according to claim 11 , further comprising:

making the plurality of hold circuits hold the first reference voltage substantially at the same time; and

A/D converting output voltages from the plurality of hold circuits at a different timing.

14. The test method according to claim 11 , further comprising executing A/D conversion on at least one of the first and the second reference voltages for a plurality of times with time intervals therebetween.

15. The test method according to claim 11 , further comprising judging whether or not the A/D conversion circuit that is checked is a non-defective product, based on a result of comparing digital values that are generated by the A/D conversion with expectation values.

16. An electronic device, comprising:

one or more signal input terminals, each said signal input terminal for respectively receiving an input signal voltage to be sampled;

a corresponding one or more input switches, each said input switch for selectively switching an output terminal of the input switch to one of its associated input signal voltage or a reference voltage;

a corresponding one or more hold circuits, each said hold circuit including a holding capacitor and a holding circuit input switch, the holding circuit input switch for selectively connecting an input terminal of the holding capacitor to the output terminal of the corresponding input switch;

a corresponding one or more path switches, each said path switch for selectively connecting an output terminal of the path switch to one of an output terminal of the associated holding capacitor or the output terminal of the associated input switch; and

a corresponding one or more channel switches, each said channel switch for selectively connecting the output of the associated path switch to an input terminal of an A/D conversion unit,

the one or more path switches thereby permitting the A/D conversion unit to selectively receive either signals for input into the hold circuits or signals as output from the hold circuits.

17. The electronic device of claim 16 , further comprising:

the A/D conversion unit for A/D converting signals as received from the channel switches; and

a memory device for storing values of the A/D converted signals.

18. The electronic device of claim 17 , further comprising

a control circuit for selectively operating said one or more input switches, said one or more holding circuit input switches, said one or more path switches, and said one or more channel switches;

a CPU for instructing an execution of A/D conversions; and

a time measurement circuit for measuring times, based on instructions from the CPU.

19. The electronic device of claim 16 , as comprising a motor control unit, the one or more signal input terminals receiving voltages associated with controlling said motor.

20. The electronic device of claim 16 , as selectively capable to test any of:

an operation preceding each hold circuit;

a retention characteristic of each hold circuit; and

an operation of the A/D conversion unit.