Analog multiplexers can vary in channel number and characteristics such as switching speed and channel resistance. However, the value of switched voltages for ICs is limited by supply voltages, which are usually ±15 V or less.
Designers can increase this voltage range by making proportional changes to both supply voltages, keeping a net difference of 30 V between positive and negative voltages. Such a multiplexer consists of the switches and control logic in each channel used in a sample-hold device.1
Figure 1 shows the circuit details for the first channel of a multichannel multiplexer (Fig. 1). It consists of switches S1-1, S2-1, and S3-1 and control logic CL-1 and CL-2. Initially, S1-1 and S3-1 are closed and S2-1 is open, with a low control signal, D1. For D1 high, the positions of all three switches are reversed.
This multiplexer does not have an operational amplifier, so voltage conversion is handled by a special device called a power-supply regulator (PSR). PSR-1 through PSR-n provide proportional changes to the supply voltages for S1-1, S2-1, and CL1, etc. The regulation depends on the value and polarity of input voltage VIN-1. PSR-OUT makes proportional changes to the supply voltages for S3-1 through S3-n and CL2-1 through CL2-n. This regulation depends on the value and polarity of output voltage VOUT. The PSR circuit (Fig. 2) consists of:
- A resistor divider with equal resistors R1, R2, R3, and R4
- Switch Q1 with resistors R5 and R7
- Switch Q2 with resistors R6 and R8
- Two common-drain or source followers2 Q3 and Q4 with source resistors R9 and R10
- Limiter diodes D1 and D2 with resistors R11 and R12 on the follower gates
Output voltages Va and Vb have initial values of +15 V and –15 V with supply voltages ±30 V and VIN = 0. At these values, Vc = 0. Switches Q1 and Q2 are closed by gate voltages Vg1 and Vg2 from dividers R5/R7 and R6/R8. To find the limiting starting voltages for these switches, add Vg1 and Vg2 to the threshold voltages for these switches: Vg1 + Vth1 and Vg2 + Vth2. If you increase VIN to a value greater than Vg1 + Vth1, switch Q1 will open; likewise for Q2. Figure 3 shows the voltage range for Vc.
Source followers Q3 and Q4 and diode limiters D1 and D2 permit input signals of different polarity. Q3 operates with positive input voltage and Q4 operates with negative voltage. When Q3’s positive output becomes more than Vg1 + Vth1, Q1 opens and Vc is a positive voltage. Similarly, with a negative input voltage on Q4, Q2 defines voltage Vc as negative. This changing of Vc creates a proportional change in Va and Vb.
VIN can range up to ±25 V. Va can change from +3.0 V to +27.0 V and Vb can change from –27 V to –3.0 V, maintaining a range of 30 V between them. Designers can further increase the output voltage by using additional followers. Note that all of the field transistors in the circuit are enhancement-mode MOSFETs.
The circuit will operate best if resistors R1 through R4 are at least 100 kΩ and resistors R9 and R10 are at least 1 MΩ. This yields an output resistance for the source followers in the range of 0.2 to 0.3 kΩ and ensures the least loss of transmission voltage Vc.
The multiplexer is reversible and can operate as a demultiplexer.
1. Velikson, Y., “Increase The Range Of Memorized Voltage For Sample-And-Hold Device,” EDN, Jan. 22, 2009.
2. Tietze, U. and Ch. Schenk, Electronic Circuits, 2nd ed., Springer-Verlag, Berlin 2008, p. 252.