Analog-to-digital converter with input signal range greater than supply voltage and extended dynamic range

[0001] This application claims priority under 35 USC § 119 (e) (1) of provisional application No. 60/659,705 filed Mar. 08, 2005.

[0002] The present invention relates to electronic circuitry and, in particular, to an analog-to-digital converter with input signal range greater than supply voltage and extended dynamic range.

[0003] Highly integrated power management applications often require the ability to measure voltage quantities that exceed the supply voltage in magnitude. This is primarily due to a basic need to maximize efficiency by running the power management IC on as low a supply voltage as possible, while still maintaining the ability to sample and measure quantities from the surroundings that could well exceed the battery voltage.

[0004] The problem can be defined as follows: Assume that there is a low power N bit SAR ADC working from a supply voltage Vdd and with a reference voltage equal to V_ref. V_ref is usually equal to or slightly less than Vdd to maximize the input signal range of the ADC. The objective is to create an N+1 bit ADC which is capable of converting an input signal range from 0V to 2V_ref. If 2V_ref happens to be greater than Vdd then this would present a problem. First of all, a reference voltage equal to 2 V_ref has to be generated from Vdd. This would mean that a power-hungry charge pump would have to be built to create a high enough voltage from which this new reference voltage can be derived. Furthermore, the charge pump would have to bias all the complementary switches (transmission gates) to eliminate the forward biasing of any body diodes in the transmission gate switches. Building a charge pump also increases the noise on this desired 2V_ref reference, and an extra pin might be required for the charge pump's storage capacitor. Furthermore, the charge pump approach does not increase the effective number of bits (ENOB) by an additional bit and doubling the input signal range does not buy you an increase in ENOB, an often-desired thing when the input signal is increased. Other solutions that might involve resistor based voltage division to divide the input signal down to the 0 to V_ref range would mean loading the input and possibly slowing down the conversion rate for resistor values that are high. This attenuation mechanism would render difficult an increase in the dynamic range, since the input signal gets divided down by the attenuation factor.

[0005] An analog-to-digital converter device capable of measuring inputs beyond a supply voltage including: an N bit analog-to-digital converter powered by a supply voltage and a reference voltage; a range resolution stage capable of receiving inputs at higher voltages than the supply voltage, providing an input to the analog-to-digital converter, and outputting a logic value of one for the N+1th bit in response to an input signal higher than the reference voltage; and a bootstrapped input multiplexer stage for connecting low voltage input signals directly to the analog-to-digital converter and for connecting input signals that can exceed the supply voltage to the range resolution stage.

[0010] An analog-to-digital converter (ADC) topology, according to the present invention, capable of measuring inputs beyond the supply voltage is presented. In this topology the core of the ADC runs from a low power supply (thus consuming less power) while the input signal range is extended well above the supply voltage.

[0011] The present invention uses a power efficient way for extending the input signal range and effective number of bits that an ADC can provide. In other words an N bit ADC that operates on a voltage reference that is less than the supply voltage, and with an input signal range that is less than the supply voltage can be expanded to an N+1 bit ADC operating from the same power supply and reference but with an expanded input signal range that goes beyond the supply voltage. This expansion happens with minimal addition to power consumption and without any attenuation of the input signal.

[0012] The advantages are: 1. Increased effective number of bits the converter can provide; 2. Expanded Range for the input signal; 3. Super power-efficient operation, that is small compared to a similar ADC with the same effective number of bits but operating at a higher supply voltage; and 4. Economic to manufacture.

[0013] The topology of the present invention allows for extending the input signal range of the ADC beyond the supply voltage with minimal additional power consumption. Furthermore, the topology gives an extra bit in resolution transforming an N bit ADC to an N+1 bit ADC. The additional modules which allow for this expansion in the input signal range and effective number of bits of an ADC are small in size and present a small overhead in terms of die area. Furthermore, the performance gains are outstanding given that the input signal range is expanded beyond the supply voltage with which the ADC core runs.

[0014] The present invention provides a robust power-efficient way of expanding the input signal range of an ADC from reference voltage V_ref to 2V_ref (two times reference voltage), while at the same time increasing the ADC ENOB (effective number of bits) by 1 bit from N bits to N+1 bits.

[0015] FIG. 1 shows a block diagram of a preferred embodiment ADC topology. The topology consists of an N bit SAR ADC 20 powered by a supply voltage Vdd and with a reference voltage V_ref; a bootstrapped input multiplexer stage (decoder) 22; a range resolution stage 24; Low-Voltage input signal Channels 1–4 and High-Voltage Channels 1–4; decoder control signal Channel Select; reference voltages VREFP and VREFN, outputs N-bits and N+1's bit (MSB). The SAR ADC 20 is a standard N bit SAR ADC. The range resolution stage includes a comparator 30, a subtractor 32, and a logic gate 34.

[0016] The bootstrapped inputs (High-Voltage Channel 1-High-Voltage Channel 4) are based on a bootstrapped switch capable of switching in inputs at higher voltages than supply voltage Vdd without turning on any body diodes. This is achieved at negligible power consumption levels. The range resolution stage 24 does the following: If the input signal is between a voltage level of 0 and V_ref then that signal is directly fed to the N-bit ADC 20. If, on the other hand, the input signal is greater than voltage V_ref then V_ref gets subtracted from the signal before it is fed to the SAR ADC 20. This range resolution decision results in an extra bit of information and has the effect of creating two input ranges each of which is equal to V_ref in magnitude. This input range folding effect is further illustrated in FIGS. 2 and 3. FIG. 2 shows the range resolution stage 24 and a scale of the ADC output codes from zero to 2V_ref. FIG. 3 shows a plot of ADC output code versus the input voltage.

[0017] Given the above information and the concept of input signal range folding, the following problem arises: For values of V_in that are greater than V_ref, a solution to precisely subtract V_ref from the input voltage within less than ½ LSB resolution is needed. The subtraction needs to take place without the use of active circuitry (that would have to run on a higher supply voltage) to take out V_ref from V_in. The solution is presented in the passive subtractor block shown in FIG. 4. The passive subtractor shown in FIG. 4 includes And gates 40 and 42; inverter 44; capacitors CIN and CSAR; switches 46 and 48; most significant bit MSB; most significant bit ready signal MSB Ready; clock signals PHI2, PHI1Z, and PHI1P; high-voltage input signal; and output node. Inverter 44 and And gate 40 form a switchable reference voltage device. The passive subtractor serves as the subtractor block 32 shown in FIG. 1.

[0018] The passive subtractor uses a purely passive subtraction technique and requires a few clock cycles to complete the subtraction. The mechanics of its workings are as follows: If the range resolution block decides that the input voltage is greater than voltage V_ref, the bottom plate of capacitor C_in is switched to voltage V_ref (the voltage on node VREFP) instead of ground (voltage on node VREFN). When clock signal PHI1P is high the input voltage is being sampled on the top plate of capacitor C_in. When clock signal PHIlP goes low and clock signal PHI2 goes high the bottom plate of capacitor C_in is switched to ground while the top plate of capacitor C_in gets shorted to capacitor C_SAR (the input capacitor of the SAR ADC). Switch 48 (connected between capacitors C_in and C_SAR) and switch 46 (connected between the high voltage input and capacitor C_in) are implemented as bootstrapped NMOS switches. After a few clock cycles pass, the voltage on capacitor CSAR will settle to within ½ LSB of resolution after which the ADC switches from the sample mode to convert mode to convert the signal. The resulting N bits from this conversion, in addition to the extra bit generated by the range resolution stage add up to an N+1 bit result, while the range folding expands the input range from (0 to V_ref) to (0 to 2V_ref). The range resolution stage comparator consumes much less current than the SAR main comparator. This is the only place where static current gets added to the overall current budget due to the dc biasing of the comparator. This additional current, however, is small and does not increase the power consumption by much. The bootstrapped switches and the passive subtractor blocks consume no static power, and the dynamic power they consume due to switching is negligible.

[0019] The preferred embodiment ADC topology is capable of resolving signals beyond the supply voltage. This topology is power efficient and increases the effective number of bits as the input signal range is expanded. The modules required to expand the dynamic range of an ADC according to this topology are small in size. The topology is robust and easily manufactured.

[0020] While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.