4-261
Theory of Operation
The HI-5701 is a 6-bit analog-to-digital converter based on a
parallel CMOS “flash” architecture. This flash technique is an
extremely fast method of A/D conversion because all bit
decisions are made simultaneously. In all, 64 comparators
are used in the HI-5701; 63 comparators to encode the
output word, plus an additional comparator to detect an
overflow condition.
The CMOS HI-5701 works by alternately switching between
a “Sample” mode and an “Auto Balance” mode. Splitting up
the comparison process in this CMOS technique offers a
number of significant advantages. The offset voltage of each
CMOS comparator is dynamically canceled with each
conversion cycle such that offset voltage drift is virtually
eliminated during operation. The block diagram and timing
diagram illustrate how the HI-5701 CMOS flash converter
operates.
The input clock which controls the operation of the HI-5701
is first split into a non-inver ting φ1 clock and an inver ting φ2
clock. These two clocks, in turn, synchronize all internal
timing of analog switches and control logic within the
converter.
In the “Auto Balance” mode (φ1), all φ1 switches close and
φ2 switches open. The output of each comparator is
momentarily tied to its own input, self-biasing the comparator
midway between VSS and VDD and presenting a low
impedance to a small input capacitor. Each capacitor, in
turn, is connected to a reference voltage tap from the
resistor ladder. The Auto Balance mode quickly precharges
all 64 input capacitors between the self-bias voltage and
each respective tap voltage.
In the “Sample” mode (φ2), all φ1 switches open and φ2
switches close. This places each comparator in a sensitive
high gain amplifier configuration. In this open loop state, the
input impedance is very high and any small voltage shift at
the input will drive the output either high or low. The φ2 state
also switches each input capacitor from its reference tap to
the input signal. This instantly transfers any voltage
diff erence between the ref erence tap and input voltage to the
comparator input. All 64 comparators are thus driven
simultaneously to a defined logic state. For example, if the
input voltage is at mid-scale, capacitors precharged near
zero during φ1 will push comparator inputs higher than the
self bias voltage at φ2; capacitors precharged near the
reference voltage push the respective comparator inputs
lower than the bias point. In general, all capacitors
precharged by taps above the input voltage force a “low”
voltage at comparator inputs; those precharged below the
input voltage force “high” inputs at the comparators.
During the next φ1 state, comparator output data is
latched into the encoder logic block and the first stage of
encoding takes place. The following φ2 state completes
the encoding process. The 6 data bits (plus overflow bit)
are latched into the output flip-flops at the next falling
clock edge. The Overflow bit is set if the input voltage
exceeds VREF+-1
/
2
LSB. The output bus may be either
enabled or disabled according to the state of CE1 and
CE2 (See Table 2). When disabled, output bits assume a
high impedance state.
As shown in the timing diagram, the digital output word
becomes valid after the second φ1 state. There is thus a one
and a half cycle pipeline delay between input sample and
digital output. “Data Output Delay” time indicates the slight
time dela y f or data to become v alid at the end of the φ1 state .
Refer to the Glossary of Terms for other definitions.
TABLE 1. PIN DESCRIPTIONS
PIN # NAME DESCRIPTION
1 D5 Bit 6, Output (MSB).
2 OVF Overflow, Output.
3V
SS Digital Ground.
4 NC No Connection.
5 CE2 Three-State Output Enable Input, Active
High (See Table 2).
6CE1 Three-State Output Enable Input, Active
Low (See Table 2).
7 CLK Clock Input.
8 PHASE Sample Clock Phase Control Input.
When Phase is Low, Sample Unknown
(φ1) Occurs When the Clock is Low and
Auto Balance (φ2) Occurs When the
Clock is High (See Text).
9V
REF+ Reference Voltage Positive Input.
10 VREF- Reference Voltage Negative Input.
11 VIN Analog Signal Input.
12 VDD Power Supply, +5V.
13 D0 Bit 1, Output (LSB).
14 D1 Bit 2, Output.
15 D2 Bit 3, Output.
16 1/2 R2 Reference Ladder Midpoint.
17 D3 Bit 4, Output.
18 D4 Bit 5, Output.
TABLE 2. CHIP ENABLE TRUTH TABLE
CE1 CE2 D0 - D5 OVF
0 1 Valid Valid
1 1 Three-State Valid
X 0 Three-State Three-State
X = Don’t Care
HI-5701