VP211
Device Description
The VP211 is a dual 90MHz 6-bit ADC system, (see
Fig.2). Included on chip is a high bandwidth ADC driver
amplifier, a 6-bit analog to digital converter, latches and TTL
compatible data outputs. The VP211 also has the necessary
bias voltages for the reference resistor chain in the ‘flash’
architecture of the ADC.
VRM
Analog Input
The analog inputs, (VIN A,B) are A.C. coupled into the
non-inverting input of the ADC driver amplifiers, which
provide the necessary bandwidth, gain, offset and low
impedance required to drive the ADC. The amplifier has
been designed so that an input of 0 volts will produce an
output level equal to the voltage present at the middle of the
ADC resistor chain, VRM (3.00V typ.). This is achieved by an
internal feedback loop within each amplifier which compares
the amplifier output with VRM, (see Fig.3). This voltage will
produce a transition binary code of 011111 to 100000 at the
output of the ADC.
INPUT
SIGNAL
C
C
ADC DRIVER
AMP
TO ADC
DC SHIFT
COMP_(Q,I)
C
COMP
Reference Voltage
An on chip band gap voltage reference circuit combined
with two op-amps provides all the necessary bias voltages
for the ADC reference resistor chain, bottom (VRB), middle
(VRM) and top(VRT). VRB, VRM and VRT have been
brought out to pins 12, 9 and 4 respectively and should be
decoupled with 100nF capacitors close to the package pins.
Fig.3 DC offset internal feedback loop
Digital Interface
The TTL data output pins, (DA0-DA5) and (DB0-DB5),
have been optimized to interface with devices in close
proximity to the VP211 and are designed to provide
satisfactory logic levels at speeds up to 90MHz into a fanout
of one and a total load capacitance of 10pF. All data outputs
should have approximately equivalent loading to ensure
proper setup and hold times. For capacitive loads in excess
of 10pF, output buffers are recommended.
ADC Circuit
The VP211 employs a ‘flash’ architecture consisting of a
reference resistor chain, an array of 64 comparators,
encoding logic and a 6-bit latch. The 63 reference levels
generated by the resistor chain are compared with the
analog output signal from the ADC driver amplifier using the
comparator array. This produces a thermometer code which
the encoding logic converts into a 6-bit word.
Clock Interface
The clock signal to the ADC synchronizes the sampling,
conversion and output stages of the device as shown in the
timing diagram (see Fig.4). The output of the ADC driver amp
is sampled when the comparator array is latched on the rising
edge of the input clock. Data is then presented to the TTL data
outputs and latched on the falling edge of the input clock.
V ref.
VIN(A,B)
Comparator
Latch
C
L
τ
1
Data Out
Clock to ADC
N-1
N
N+1
VIN(A,B)
Tpw 1
CLKIN
Tpw 0
Data Outputs
N-1
N
Tsu
Fig.4 System timing diagram
N+1
T
Hold
TTL
Threshold
5
MITEL [ MITEL NETWORKS CORPORATION ]
