MPC507AP datasheet(5/19 Pages) TI

5 / 19 page

MPC506A, MPC507A

SBFS018A

www.ti.com

DISCUSSION OF

SPECIFICATIONS

DC CHARACTERISTICS

The static or dc transfer accuracy of transmitting the multi-

plexer input voltage to the output depends on the channel

ON resistance (R

ON), the load impedance, the source imped-

ance, the load bias current and the multiplexer leakage

current.

Single-Ended Multiplexer Static Accuracy

The major contributors to static transfer accuracy for single-

ended multiplexers are:

Source resistance loading error

Multiplexer ON resistance error

dc offset error caused by both load bias current and

multiplexer leakage current.

Resistive Loading Errors

The source and load impedances will determine the input

resistive loading errors. To minimize these errors:

• Keep loading impedance as high as possible. This mini-

mizes the resistive loading effects of the source resistance

and multiplexer ON resistance. As a guideline, load

impedance of 108

Ω or greater will keep resistive loading

errors to 0.002% or less for 1000

Ω source impedances. A

106

Ω load impedance will increase source loading error

to 0.2% or more.

• Use sources with impedances as low as possible. A

1000

Ω source resistance will present less than 0.001%

loading error and 10k

Ω source resistance will increase

source loading error to 0.01% with a 108 load impedance.

Input resistive loading errors are determined by the follow-

ing relationship (see Figure 1).

Input Offset Voltage

Bias current generates an input OFFSET voltage as a result

of the IR drop across the multiplexer ON resistance and

source resistance. A load bias current of 10nA will generate

an offset voltage of 20

µV if a 1kΩ source is used. In general,

for the MPC506A, the OFFSET voltage at the output is

determined by:

OFFSET = (IB + IL) (RON + RS)

where I

B = Bias current of device multiplexer is driving

L = Multiplexer leakage current

ON = Multiplexer ON resistance

S = Source resistance

Differential Multiplexer Static Accuracy

Static accuracy errors in a differential multiplexer are diffi-

cult to control, especially when it is used for multiplexing

low-level signals with full-scale ranges of 10mV to 100mV.

The matching properties of the multiplexer, source and

output load play a very important part in determining the

transfer accuracy of the multiplexer. The source impedance

unbalance, common-mode impedance, load bias current

mismatch, load differential impedance mismatch, and com-

mon-mode impedance of the load all contribute errors to the

multiplexer. The multiplexer ON resistance mismatch, leak-

age current mismatch and ON resistance also contribute to

differential errors.

Referring to Figure 2, the effects of these errors can be

minimized by following the general guidelines described in

this section, especially for low-level multiplexing applica-

tions.

FIGURE 1. MPC506A Static Accuracy Equivalent Circuit.

Source and Multiplexer Resistive Loading Error

∈+

()

SON

100

where R

S = source resistance

L = load resistance

ON = multiplexer ON resistance

S16

OFF

S16

Measured

Voltage

BIAS

FIGURE 2. MPC507A Static Accuracy Equivalent Circuit.

S8A

S8B

OFF8A

OFF8B

S1A

S1B

ON1A

ON1B

Cd/2

Rd/2

BIAS A

BIAS B

CM8

CM1

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