ADA4500-2

Data Sheet

Rev. A | Page 22 of 24

APPLICATIONS INFORMATION

RESISTANCE AND CAPACITANCE SENSOR CIRCUIT

The application shown in Figure 65 generates a square-wave

(1)

U1A takes advantage of the high input impedance and large rail-

to-rail input dynamic range of the ADA4500-2 to measure a

U1B is used as a comparator; with the noninverting input

output swinging from rail to rail. Because the accuracy of the

circuit depends on the propagation time through the amplifers,

the fast recovery of U1B from the output overload conditions

makes it ideal for this application.

U1A

ADA4500-2

U1B

ADA4500-2

OUTPUT

R1

10kΩ

R3

100kΩ

R2

100kΩ

Rx

Cx

Figure 65. A Resistance/Capacitance Sensor

ADAPTIVE SINGLE-ENDED-TO-DIFFERENTIAL

SIGNAL CONVERTER

The Challenge

When designing a signal path in systems that have a single voltage

supply, the biggest challenge is how to represent the full range of

an input signal that may have positive, zero, and negative values.

By including zero in the output, the output signal must go

completely to ground, which single-supply amplifiers cannot do.

Converting the single-ended input signal to a differential signal

(through a single-ended-to-differential signal converter circuit)

allows zero to be represented as the positive and negative outputs

being equal, requiring neither amplifier to go to ground.

There are other benefits of the single-ended-to-differential signal

conversion, such as doubling the amplitude of the signal for

better signal-to-noise ratio, rejecting common-mode noise, and

driving the input of a high precision differential ADC.

In addition to converting to a differential signal, the circuit must set

the common-mode dc level of its output to a level that gives the ac

signal maximum swing at the load (like the input to an ADC).

Three key challenges are encountered often when designing a

single-ended-to-differential signal converter circuit with a

single supply:

•

When the supply is limited to a single voltage, the input

signal level to the circuit is generally limited to operate

on the input dynamic range can require attenuation and/or

level-shifting of the source signal before it even gets to the

single-ended-to-differential signal converter. This results in

reduced signal-to-noise ratio (SNR) and additional error.

•

The dc part of the input signal, on which the ac signal rides, is

generally not known during system operation. For example, if

multiple input signals from varying sources are multiplexed

into the single-ended-to-differential signal converter circuit,

each one could have a different dc level. Accommodating

multiple dc input levels means that the system design must

compromise the maximum allowed peak voltage of the ac

part of the input so that it does not clip against the rails.

•

The system processor does not know what the dc level is of the

original signal so it cannot make adjustments accordingly.

The Solution

These challenges are solved with the adaptive single-ended to

differential converter shown in Figure 66. This circuit operates

off a single supply from 2.7 V to 5.5 V, it automatically adjusts the

dc common mode of the output to a desired level, and it provides

the ability to measure the dc component of the input signal. This

U1B integrates that signal and feeds the integrated (dc) voltage

and R11 are set to equal the impedance of the resistors R8 and R9

for a matched ac response and for balancing the effects of the

bias current.

The input frequency can range from 10 Hz to 1 MHz. Peak-to-peak

voltage can take a common mode from as high as +7.5 V and as

low as −2.5 V with a signal amplitude of 5 V p-p. The wide range of

large as the supply, eliminates the need to reduce the amplitude

of the input signal and sacrifice SNR. When measuring both the

ac and the dc parts of the signal, a capacitor cannot be in the signal

path. Figure 66 shows examples of the voltage ranges of the single-

ended-to-differential signal converter circuit.