What are the considerations of using a common anode 7-segment display over a common cathode?

Question

Common anode:

Do I need resistors here? Where would I connect them?

Common cathode:

Common anode seems simpler circuit wise - but I like the idea that "1" represents "ON" that common cathode has.

Extra question: Could I use a NOT gate on the common anode to make the inputs "1" turn the LEDs on?

Answer 1

What are the considerations of using a common anode 7-segment display over a common cathode?

The only consideration is which chip you're using. If you haven't picked a chip, then the consideration is the cost and availability of parts. If you're spinning your own circuit (no specialized IC) using external transistors to carry the current, then it depends on whether you're driving the segments individually or in a matrix. If individually, common anode is easier because N-polarity devices (N-channel MOSFETs, NPN BJTs) are more common and perform better for a given size and price. If in a matrix, it depends on whether you're scanning the anodes or the cathodes.

Do I need resistors here? Where would I connect them?

Yes. 7 resistors, such as 330 Ohm, each in series with each line between the IC and the display. But be aware that the 7447 IC is only capable of driving a few mA.

_{(Image source: Answer from Sachin Pathave to the question "How can we connect 7 segment display with IC 7447?" on Quora)}

Could I use a NOT gate on the common anode to make the inputs "1" turn the LEDs on?

Yes. Again, you need resistors and the inverter can only drive a few mA. That's fine for a small display with individually driven lines. It's not enough for a large display and certainly not for a scanned display connected in a matrix.

Answer 2

Regarding 7447 7-segment driver:

Do I need resistors here? Where would I connect them?

I've never seen this chip used without current limiting resistors, when driving LED seven-segment display. Generally, each of a,b,c,d,e,f,g output pins gets a similar-value series resistor to limit current.

^{simulate this circuit – Schematic created using CircuitLab}

Since this chip wants a +5v DC supply, LED common-anode pin is often connected to +5V as well. This is convenient, but not required. Each LED of a common-anode array needs at least a few volts to even begin to light up. The 7447 internal transistor adds a few hundred millivolts to this, leaving perhaps 2.8V for each series resistor to soak-up.
If you decide that 5mA LED current provides a bright-enough display, then each of seven series resistors would be 2.8V/5mA = 560 ohms. Some LEDs are super-bright while others (perhaps older) are not. Brilliance is adjusted by substituting other resistor values - brilliance is almost proportional to current.

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The 4511 seven-segment driver is a different animal, not only because it is meant for common-cathode displays, but because it is based on hybrid CMOS/BJT logic. But the same current-limiting method is used to drive LED displays (from a data sheet acquired from Harris Semiconductor SCHS072B - Revised July 2003):

Extra question: Could I use a NOT gate on the common anode to make the inputs "1" turn the LEDs on?

Inverter at the four 7447 BCD logic inputs? Simply - No!

Seven inverters at all seven 7447 outputs? Possibly, but a well-engineered circuit would include seven pull-up resistors to +5v at 7447 output pins.
You couldn't drive a common-anode LED display from inverter outputs, but you could drive a common-cathode LED display through seven series resistors from inverter outputs. A many-parts, awkward solution. Many TTL inverters only weakly provide drive current to a common-cathode LED display. You should carefully search for an inverter meant for high-side driving. The 4511 is designed for good high-side drive.

Answer 3

The fact that you can find both types of displays on the market - still - shows that there isn't agreement on which is better.

For a single digit, it likely doesn't matter (CA or CC) so long as the driver you intend to use matches what the display requires.

If you're driving digits from an MCU directly without multiplexing, it could be argued that CC is more logical, as the drives are active-high. On the other hand, CA allows the MCU to sink current - this is preferred if you're using an 8051 for example.

If your LEDs needed higher voltage (say, your LEDs are blue or white and need 5V and you only have 3.3V logic) common anode with open-drain / open collector drivers might be best. Many driver ICs that have internal current limiting work best with this.

For multiplexed displays, it could be argued that common-cathode is a bit better, as the per-digit switching is on the low side and can thus use an NPN or n-FET per digit with an active-high control to the driver.

But I've seen common-anode muxed displays too, even built one using using the SAA1064, which employs NPN emitter followers for the digit drives.