Increase the range of memorized voltage for a sample-and-hold device

Sample-and-hold devices find use in front of ADCs. The basic sample-and-hold circuit comprises two op amps, A₁ and A₂; a switch, S₁; and a capacitor, C₁ (Figure 1). For many low-power op amps, the values of the input and output voltages can be only ±10 to ±14V using a standard ±15V power supply. Enabling these devices to handle greater voltage can significantly improve the resolution of an ADC.

You can increase the memorized voltage that amplifiers A₁ and A₂ can reach by using a variable power supply (reference 1 and reference 2). This approach places additional voltage requirements on S₁, however. To continue using switches with the same range as the original, you must add two switches and independent control-logic blocks, CL₁ and CL₂, for switches S₁, S₂, and S₃ (Figure 2). The two parts of the circuit may have independent power supplies. You apply the same variable voltages to amplifiers A₁ and A₂ as you do to control-logic blocks CL₁ and CL₂, respectively. When S₁ and S₃ are closed, S₂ is open, and vice versa.

The resulting circuit keeps the voltages connected to the gate and substrate for the MOS transistors of each switch within the desired 30V range (Figure 3). (You derive this value from the sum of absolute-voltage values: |V₁|+|V₂| and |V₃|=|V₄|.) Voltages V₁ and –V₂ connect to amplifier A₁, control-logic block CL₁, and the substrates of the transistors of switches S₁ and S₂. Voltages V₃ and –V₄ connect to amplifier A₂, control-logic block CL₂, and the substrates of the transistors of switch S₃.

You create the changing voltages of V₁ and V₂ using resistor dividers R₅ and R₆ and R₇ and R₈, which connect to the 30 and the –30V power supplies and the output of amplifier follower A₁ (Figure 3). Transistors Q₁ and Q₂ create the change to the power supply of amplifier A₁. Voltages V₁ and V₂ also supply power to control-logic block CL₁ and the substrates of the transistors of switches S₁ and S₂. CL₁ comprises transistors Q₁₁, Q₁₂, Q₁₅, and Q₁₆. It creates a control signal for gates Q₅ and Q₆ of switch S₁ and the inverse signal for gates Q₈ and Q₉ of S₂.

Resistor dividers R₉ and R₁₀ and R₁₁ and R₁₂ connect to the 30 and the –30V power supplies, and the output of amplifier follower A₂ creates the changing voltages V₃ and V₄. Transistors Q₃ and Q₄ create the change to the power supply of amplifier A₂. Voltages V₃ and V₄ also supply power to control-logic block CL₂ and the substrates of the transistors of switch S₃. CL₂ is made up of transistors Q₁₃, Q₁₄, Q₁₇, and Q₁₈. It creates a control signal for gates Q₇ and Q₁₀ of switch S₃. Transistors Q₅ through Q₁₀ and Q₁₁ through Q₁₈ of CL₁ and CL₂, respectively, are complementary pairs of MOS logic transistors.