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Data Sheet

AD7877

It is only during the sample or acquisition phase of the ADC

operation of the AD7877 that noise from the LCD screen has an

effect on the ADC measurements. During the hold or

conversion phase, the noise has no effect, because the voltage at

the input of the ADC has already been acquired. Therefore, to

minimize the effect of noise on the touch screen measurements,

the ADC acquisition phase should be halted.

The LCD control signal should be applied to the STOPACQ pin.

To ensure that acquisition never occurs during the noisy period

when the LCD signal is active, the AD7877 monitors this signal.

No acquisitions take place when the control signal is active. Any

acquisition that is in progress when the signal becomes active is

aborted and restarts when the signal becomes inactive again.

To accommodate signals of different polarities on the

STOPACQ pin, a user-programmable register bit is used to

indicate whether the signal is active high or low. The POL bit

is Bit 3 in Control Register 2, Address 0x02. Setting POL to 1

indicates that the signal on STOPACQ is active high; setting

POL to 0 indicates that it is active low. POL defaults to 0 on

power-up. To disable monitoring of STOPACQ, the pin should

be tied low if POL = 1, or tied high if POL = 0. Under no

circumstances should the pin be left floating.

The signal on STOPACQ has no effect while the ADC is in

conversion mode, or during the first conversion delay time.

(See the Control Registers section for details on the first

conversion delay.)

When enabled, the STOPACQ monitoring function is imple-

mented on all input channels to the ADC: AUX1, AUX2, BAT1,

BAT2, TEMP1, and TEMP2, as well as on the touch screen

input channels.

TEMPERATURE MEASUREMENT

Two temperature measurement options are available on the

AD7877: the single conversion method and the differential

conversion method. The single conversion method requires

only a single measurement on ADC Channel 1000b. Whereas

differential conversion requires two measurements, one on

ADC Channel 1000b and a second on ADC Channel 1001b.

The results are stored in the results registers with Address 11000b

(TEMP1) and Address 11001b (TEMP2). The AD7877 does not

provide an explicit output of the temperature reading; the

system must perform some external calculations. Both methods

are based on an on-chip diode measurement.

Single Conversion Method

The single conversion method makes use of the fact that the

temperature coefficient of a silicon diode is approximately

−2.1 mV/°C. However, this small change is superimposed on

the diode forward voltage, which can have a wide tolerance. It

is, therefore, necessary to calibrate by measuring the diode

voltage at a known temperature to provide a baseline from

which the change in forward voltage with temperature can be

measured. This method provides a resolution of approximately

0.3°C and a predicted accuracy of ±2.5°C.

The temperature limit comparison is performed on the result in

the TEMP1 results register, which is simply the measurement of

the diode forward voltage. The values programmed into the high

and low limits should be referenced to the calibrated diode for-

ward voltage to make accurate limit comparisons. An example

is shown in the Limit Comparison section.

Differential Conversion Method

The differential conversion method is a 2-point measurement.

The first measurement is performed with a fixed bias current

into a diode (when the TEMP1 channel is selected), and the

second measurement is performed with a fixed multiple of the

bias current into the same diode (when the TEMP2 channel is

selected). The voltage difference in the diode readings is pro-

portional to absolute temperature and is given by the following

formula:

∆VBE = (kT/q) × (ln N)

(4)

where:

VBE represents the diode voltage.

N is the bias current multiple (typical value for AD7877 = 120).

k is Boltzmann’s constant.

q is the electron charge.

This method provides a resolution of approximately 1.6°C, and

a guaranteed accuracy of ±4°C without calibration. Determina-

tion of the N value on a part-by-part basis improves accuracy.

Assuming a current multiple of 120, which is a typical value

for the AD7877, taking Boltzmann’s constant, k = 1.38054 ×

10−23 electrons V/°K, the electron charge q = 1.602189 × 10−19,

then T, the ambient temperature in Kelvin, would be calculated

as follows:

∆VBE = (kT/q) × (ln N)

T°k = (∆VBE × q)/(k × ln N)

= (∆VBE × 1.602189 × 10−19)/(1.38054 × 10−23 × 4.65)

T°C = 2.49 × 120 × ∆VBE − 273

∆VBE is calculated from the difference in readings from the first

conversion to the second conversion. The user must perform

the calculations to get ∆VBE, and then calculate the temperature

value in degrees. Figure 33 shows a block diagram of the

temperature measurement circuit.

TEMP1

TEMP2

MUX

ADC

VBE

I20 × I

Figure 33. Block Diagram of Temperature Measurement Circuit

Rev. E | Page 17 of 45

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