ADC0820-N

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SNAS529C – JUNE 1999 – REVISED MARCH 2013

FUNCTIONAL DESCRIPTION

GENERAL OPERATION

The ADC0820-N uses two 4-bit flash A/D converters to make an 8-bit measurement (Figure 3). Each flash ADC

is made up of 15 comparators which compare the unknown input to a reference ladder to get a 4-bit result. To

take a full 8-bit reading, one flash conversion is done to provide the 4 most significant data bits (via the MS flash

ADC). Driven by the 4 MSBs, an internal DAC recreates an analog approximation of the input voltage. This

analog signal is then subtracted from the input, and the difference voltage is converted by a second 4-bit flash

ADC (the LS ADC), providing the 4 least significant bits of the output data word.

The internal DAC is actually a subsection of the MS flash converter. This is accomplished by using the same

resistor ladder for the A/D as well as for generating the DAC signal. The DAC output is actually the tap on the

resistor ladder which most closely approximates the analog input. In addition, the “sampled-data” comparators

used in the ADC0820-N provide the ability to compare the magnitudes of several analog signals simultaneously,

without using input summing amplifiers. This is especially useful in the LS flash ADC, where the signal to be

converted is an analog difference.

THE SAMPLED-DATA COMPARATOR

Each comparator in the ADC0820-N consists of a CMOS inverter with a capacitively coupled input (Figure 17

Figure 18). Analog switches connect the two comparator inputs to the input capacitor (C) and also connect the

inverter's input and output. This device in effect now has one differential input pair. A comparison requires two

cycles, one for zeroing the comparator, and another for making the comparison.

In the first cycle, one input switch and the inverter's feedback switch (Figure 17) are closed. In this interval, C is

charged to the connected input (V1) less the inverter's bias voltage (VB, approximately 1.2V). In the second cycle

(Figure 18), these two switches are opened and the other (V2) input's switch is closed. The input capacitor now

subtracts its stored voltage from the second input and the difference is amplified by the inverter's open loop gain.

The inverter's input (VB′) becomes

(1)

and the output will go high or low depending on the sign of VB′−VB.

The actual circuitry used in the ADC0820-N is a simple but important expansion of the basic comparator

described above. By adding a second capacitor and another set of switches to the input (Figure 19), the scheme

can be expanded to make dual differential comparisons. In this circuit, the feedback switch and one input switch

on each capacitor (Z switches) are closed in the zeroing cycle. A comparison is then made by connecting the

second input on each capacitor and opening all of the other switches (S switches). The change in voltage at the

inverter's input, as a result of the change in charge on each input capacitor, will now depend on both input signal

differences.

• VO = VB

• V on C = V1

−VB

• CS = stray input node capacitor

• VB = inverter input bias voltage

Zeroing Phase

Figure 17. Sampled-Data Comparator

Compare Phase

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