sion, the digital output must be updated with the results of the
last bit decision, the capacitor array appropriately switched
and charged, and the input to the comparator settled to a
12-bit level all within one clock cycle.
described in the previous paragraph, voltage variation due to
the line frequency (50Hz or 60Hz), can be difficult to
remove.
The GND pin on the ADS7816 should be placed on a clean
ground point. In many cases, this will be the “analog”
ground. Avoid connecting the GND pin too close to the
grounding point for a microprocessor, microcontroller, or
digital signal processor. If needed, run a ground trace di-
rectly from the converter to the power supply connection
point. The ideal layout will include an analog ground plane
for the converter and associated analog circuitry.
The basic SAR architecture is sensitive to spikes on the
power supply, reference, and ground connections that occur
just prior to latching the comparator output. Thus, during
any single conversion for an n-bit SAR converter, there are
n “windows” in which large external transient voltages can
easily affect the conversion result. Such spikes might origi-
nate from switching power supplies, digital logic, and high
power devices, to name a few. This particular source of error
can be very difficult to track down if the glitch is almost
synchronous to the converter’s DCLOCK signal—as the
phase difference between the two changes with time and
temperature, causing sporadic misoperation.
The –In input pin should be connected directly to ground. In
those cases where the ADS7816 is a large distance from the
signal source and/or the circuit environment contains large
EMI or RFI sources, the –In input should be connected to the
ground nearest the signal source. This should be done with
a signal trace that is adjacent to the +In input trace. If
appropriate, coax cable or twisted-pair wire can be used.
With this in mind, power to the ADS7816 should be clean
and well bypassed. A 0.1µF ceramic bypass capacitor should
be placed as close to the ADS7816 package as possible. In
addition, a 1 to 10µF capacitor and a 10Ω series resistor may
be used to lowpass filter a noisy supply.
APPLICATION CIRCUITS
The reference should be similarly bypassed with a 0.1µF
capacitor. Again, a series resistor and large capacitor can be
used to lowpass filter the reference voltage. If the reference
voltage originates from an op amp, be careful that the op-
amp can drive the bypass capacitor without oscillation (the
series resistor can help in this case). Keep in mind that while
the ADS7816 draws very little current from the reference on
average, there are higher instantaneous current demands
placed on the external reference circuitry.
Figures 6, 7, and 8 show some typical application circuits for
the ADS7816. Figure 6 uses an ADS7816 and a multiplexer
to provide for a flexible data acquisition circuit. A resistor
string provides for various voltages at the multiplexer input.
The selected voltage is buffered and driven into VREF. As
shown in Figure 6, the input range of the ADS7816 is
programmable to 100mV, 200mV, 300mV, or 400mV. The
100mV range would be useful for sensors such as the
thermocouple shown.
Also, keep in mind that the ADS7816 offers no inherent
rejection of noise or voltage variation in regards to the
reference input. This is of particular concern when the
reference input is tied to the power supply. Any noise and
ripple from the supply will appear directly in the digital
results. While high frequency noise can be filtered out as
Figure 7 is more complex variation of Figure 6 with in-
creased flexibility. In this circuit, a digital signal processor
designed for audio applications is put to use in running three
ADS7816s and a DAC56. The DAC56 provides a variable
voltage for VREF—enabling the input range of the ADS7816s
to be programmed from 100mV to 3V.
+5V
+5V
+5V
R8
46kΩ
0.4V
0.3V
R7
10Ω
R9
1kΩ
R1
150kΩ
OPA237
D1
C2
0.1µF
U2
R3
500kΩ
R10
1kΩ
C1
10µF
MUX
R6
1MΩ
R2
59kΩ
VREF
0.2V
0.1V
DCLOCK
DOUT
R11
1kΩ
C3
ADS7816
TC1
A0
0.1µF
TC2
TC3
CS/SHDN
A1
Thermocouple
R12
1kΩ
U1
C4
10µF
R4
1kΩ
U3
C5
0.1µF
R5
500Ω
µP
ISO Thermal Block
3-Wire
Interface
U4
FIGURE 6. Thermocouple Application Using a MUX to Scale the Input Range of the ADS7816.
®
ADS7816
12
BB [ BURR-BROWN CORPORATION ]
