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# PROGRAMMABLE RESISTOR VOLTAGE DIVIDER THAT SAVES AREA IN THE PHYSICAL CONSTRUCTION USED IN AN INTEGRATED CIRCUIT

IP.com Disclosure Number: IPCOM000013405D
Original Publication Date: 2000-Feb-01
Included in the Prior Art Database: 2003-Jun-18
Document File: 3 page(s) / 86K

IBM

## Abstract

A very common circuit that is used on many integrated circuits is a programmable resistor voltage divider. The normal practice is to use FET 1 transistors as switches to select the resistors that are connected together to form the voltage divider. For best accuracy, the resistance of the FET switches must be much smaller than the resistors they are switching into and out of the circuit. If the resistors are of value 5 K ohms then the FET switch must be of resistance 5 ohms for an accuracy of 0.1%. An FET switch of this size can use many square microns of chip area. The circuit described below allows good accuracy without the requirement of very large FET switches. Referring to the above circuit diagram, some of the major components are identified.

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PROGRAMMABLE RESISTOR VOLTAGE DIVIDER THAT SAVES AREA IN THE PHYSICAL CONSTRUCTION USED IN AN INTEGRATED CIRCUIT

A very common circuit that is used on many integrated circuits is a
programmable resistor voltage divider. The normal practice is to use FET

1

Page 2 of 3

transistors as switches to select the resistors that are connected together to
form the voltage divider. For best accuracy, the resistance of the FET
switches must be much smaller than the resistors they are switching into and
out of the circuit. If the resistors are of value 5 K ohms then the FET switch
must be of resistance 5 ohms for an accuracy of 0.1%. An FET switch of this
size can use many square microns of chip area. The circuit described below
allows good accuracy without the requirement of very large FET switches.

Referring to the above circuit diagram, some of the major components are
identified.

1. PFET/NFET devices M24/M27, M20/M23, M96/M95, and M68/M71 are CMOS
inverters.
2. Device I37 is a CMOS NAND and inverter circuit.
3. M54/M56 and M49/M50 are NFET voltage dividers used to generate a voltage
of 1/2 the VCC value.
4. M73 and M72 are NFET transistors used to short the voltage generated by
3 above to ground.
5. NFET devices M2, M4, M5, M15, M55, and M47 are used to select the
resistors used for the voltage divider.

Looking at a classic resistor voltage divider we can look at what happens when
a resistor is inserted in series with each of the resistors.

Vo Vin

= R2

R1 + R2

If we have switches in series then; Vos

RR=Vins2 +2s

R1 + R1s + R2 + R2s

Want Vo

Vin

= Vos

Vins

R=R1 + R22 + R2s

R1 + R1s + R2 + R2s

Define as ; R1s = R1

and R2s = R...